diff --git a/Orange/projection/base.py b/Orange/projection/base.py
index b07f0cb39e1..1dae7f7b21b 100644
--- a/Orange/projection/base.py
+++ b/Orange/projection/base.py
@@ -1,3 +1,5 @@
+import warnings
+
 import copy
 import inspect
 import threading
@@ -9,6 +11,7 @@
 from Orange.data.util import SharedComputeValue, get_unique_names
 from Orange.misc.wrapper_meta import WrapperMeta
 from Orange.preprocess import RemoveNaNRows
+from Orange.util import dummy_callback, wrap_callback, OrangeDeprecationWarning
 import Orange.preprocess
 
 __all__ = ["LinearCombinationSql", "Projector", "Projection", "SklProjector",
@@ -44,17 +47,36 @@ def fit(self, X, Y=None):
         raise NotImplementedError(
             "Classes derived from Projector must overload method fit")
 
-    def __call__(self, data):
-        data = self.preprocess(data)
+    def __call__(self, data, progress_callback=None):
+        if progress_callback is None:
+            progress_callback = dummy_callback
+        progress_callback(0, "Preprocessing...")
+        try:
+            cb = wrap_callback(progress_callback, end=0.1)
+            data = self.preprocess(data, progress_callback=cb)
+        except TypeError:
+            data = self.preprocess(data)
+            warnings.warn("A keyword argument 'progress_callback' has been "
+                          "added to the preprocess() signature. Implementing "
+                          "the method without the argument is deprecated and "
+                          "will result in an error in the future.",
+                          OrangeDeprecationWarning, stacklevel=2)
         self.domain = data.domain
+        progress_callback(0.1, "Fitting...")
         clf = self.fit(data.X, data.Y)
         clf.pre_domain = data.domain
         clf.name = self.name
+        progress_callback(1)
         return clf
 
-    def preprocess(self, data):
-        for pp in self.preprocessors:
+    def preprocess(self, data, progress_callback=None):
+        if progress_callback is None:
+            progress_callback = dummy_callback
+        n_pps = len(self.preprocessors)
+        for i, pp in enumerate(self.preprocessors):
+            progress_callback(i / n_pps)
             data = pp(data)
+        progress_callback(1)
         return data
 
     # Projectors implemented using `fit` access the `domain` through the
@@ -208,8 +230,8 @@ def _get_sklparams(self, values):
             raise TypeError("Wrapper does not define '__wraps__'")
         return params
 
-    def preprocess(self, data):
-        data = super().preprocess(data)
+    def preprocess(self, data, progress_callback=None):
+        data = super().preprocess(data, progress_callback)
         if any(v.is_discrete and len(v.values) > 2
                for v in data.domain.attributes):
             raise ValueError("Wrapped scikit-learn methods do not support "
diff --git a/Orange/widgets/unsupervised/owpca.py b/Orange/widgets/unsupervised/owpca.py
index 0a917fe0f11..29611146fb2 100644
--- a/Orange/widgets/unsupervised/owpca.py
+++ b/Orange/widgets/unsupervised/owpca.py
@@ -12,6 +12,7 @@
 from Orange.preprocess import preprocess
 from Orange.projection import PCA
 from Orange.widgets import widget, gui, settings
+from Orange.widgets.utils.concurrent import ConcurrentWidgetMixin
 from Orange.widgets.utils.slidergraph import SliderGraph
 from Orange.widgets.utils.widgetpreview import WidgetPreview
 from Orange.widgets.widget import Input, Output
@@ -21,7 +22,7 @@
 LINE_NAMES = ["component variance", "cumulative variance"]
 
 
-class OWPCA(widget.OWWidget):
+class OWPCA(widget.OWWidget, ConcurrentWidgetMixin):
     name = "PCA"
     description = "Principal component analysis with a scree-diagram."
     icon = "icons/PCA.svg"
@@ -57,13 +58,13 @@ class Error(widget.OWWidget.Error):
 
     def __init__(self):
         super().__init__()
-        self.data = None
+        ConcurrentWidgetMixin.__init__(self)
 
+        self.data = None
         self._pca = None
         self._transformed = None
         self._variance_ratio = None
         self._cumulative = None
-        self._init_projector()
 
         # Components Selection
         form = QFormLayout()
@@ -114,6 +115,7 @@ def __init__(self):
 
     @Inputs.data
     def set_data(self, data):
+        self.cancel()
         self.clear_messages()
         self.clear()
         self.information()
@@ -138,12 +140,11 @@ def set_data(self, data):
                 self.clear_outputs()
                 return
 
-        self._init_projector()
-
         self.data = data
         self.fit()
 
     def fit(self):
+        self.cancel()
         self.clear()
         self.Warning.trivial_components.clear()
         if self.data is None:
@@ -151,27 +152,45 @@ def fit(self):
 
         data = self.data
 
-        if self.normalize:
-            self._pca_projector.preprocessors = \
-                self._pca_preprocessors + [preprocess.Normalize(center=False)]
-        else:
-            self._pca_projector.preprocessors = self._pca_preprocessors
+        projector = self._create_projector()
 
         if not isinstance(data, SqlTable):
-            pca = self._pca_projector(data)
-            variance_ratio = pca.explained_variance_ratio_
-            cumulative = numpy.cumsum(variance_ratio)
-
-            if numpy.isfinite(cumulative[-1]):
-                self.components_spin.setRange(0, len(cumulative))
-                self._pca = pca
-                self._variance_ratio = variance_ratio
-                self._cumulative = cumulative
-                self._setup_plot()
-            else:
-                self.Warning.trivial_components()
+            self.start(self._call_projector, data, projector)
+
+    @staticmethod
+    def _call_projector(data: Table, projector, state):
+
+        def callback(i: float, status=""):
+            state.set_progress_value(i * 100)
+            if status:
+                state.set_status(status)
+            if state.is_interruption_requested():
+                raise Exception  # pylint: disable=broad-exception-raised
+
+        return projector(data, progress_callback=callback)
+
+    def on_done(self, result):
+        pca = result
+        variance_ratio = pca.explained_variance_ratio_
+        cumulative = numpy.cumsum(variance_ratio)
+
+        if numpy.isfinite(cumulative[-1]):
+            self.components_spin.setRange(0, len(cumulative))
+            self._pca = pca
+            self._variance_ratio = variance_ratio
+            self._cumulative = cumulative
+            self._setup_plot()
+        else:
+            self.Warning.trivial_components()
+
+        self.commit.now()
 
-            self.commit.now()
+    def on_partial_result(self, result):
+        pass
+
+    def onDeleteWidget(self):
+        self.shutdown()
+        super().onDeleteWidget()
 
     def clear(self):
         self._pca = None
@@ -184,7 +203,7 @@ def clear_outputs(self):
         self.Outputs.transformed_data.send(None)
         self.Outputs.data.send(None)
         self.Outputs.components.send(None)
-        self.Outputs.pca.send(self._pca_projector)
+        self.Outputs.pca.send(self._create_projector())
 
     def _setup_plot(self):
         if self._pca is None:
@@ -251,10 +270,13 @@ def _update_normalize(self):
         if self.data is None:
             self._invalidate_selection()
 
-    def _init_projector(self):
-        self._pca_projector = PCA(n_components=MAX_COMPONENTS, random_state=0)
-        self._pca_projector.component = self.ncomponents
-        self._pca_preprocessors = PCA.preprocessors
+    def _create_projector(self):
+        projector = PCA(n_components=MAX_COMPONENTS, random_state=0)
+        projector.component = self.ncomponents  # for use as a Scorer
+        if self.normalize:
+            projector.preprocessors = \
+                PCA.preprocessors + [preprocess.Normalize(center=False)]
+        return projector
 
     def _nselected_components(self):
         """Return the number of selected components."""
@@ -338,11 +360,10 @@ def commit(self):
                 numpy.hstack((self.data.metas, transformed.X)),
                 ids=self.data.ids)
 
-        self._pca_projector.component = self.ncomponents
         self.Outputs.transformed_data.send(transformed)
         self.Outputs.components.send(components)
         self.Outputs.data.send(data)
-        self.Outputs.pca.send(self._pca_projector)
+        self.Outputs.pca.send(self._create_projector())
 
     def send_report(self):
         if self.data is None:
diff --git a/Orange/widgets/unsupervised/tests/test_owpca.py b/Orange/widgets/unsupervised/tests/test_owpca.py
index 3ea5009afe5..993871980a0 100644
--- a/Orange/widgets/unsupervised/tests/test_owpca.py
+++ b/Orange/widgets/unsupervised/tests/test_owpca.py
@@ -7,7 +7,6 @@
 
 from Orange.data import Table, Domain, ContinuousVariable, TimeVariable
 from Orange.preprocess import preprocess
-from Orange.preprocess.preprocess import Normalize
 from Orange.widgets.tests.base import WidgetTest
 from Orange.widgets.tests.utils import table_dense_sparse, possible_duplicate_table
 from Orange.widgets.unsupervised.owpca import OWPCA
@@ -33,7 +32,7 @@ def test_constant_data(self):
         # Ignore the warning: the test checks whether the widget shows
         # Warning.trivial_components when this happens
         with np.errstate(invalid="ignore"):
-            self.send_signal(self.widget.Inputs.data, data)
+            self.send_signal(self.widget.Inputs.data, data, wait=5000)
         self.assertTrue(self.widget.Warning.trivial_components.is_shown())
         self.assertIsNone(self.get_output(self.widget.Outputs.transformed_data))
         self.assertIsNone(self.get_output(self.widget.Outputs.components))
@@ -56,12 +55,12 @@ def test_limit_components(self):
         X = np.random.RandomState(0).rand(101, 101)
         data = Table.from_numpy(None, X)
         self.widget.ncomponents = 100
-        self.send_signal(self.widget.Inputs.data, data)
+        self.send_signal(self.widget.Inputs.data, data, wait=5000)
         tran = self.get_output(self.widget.Outputs.transformed_data)
         self.assertEqual(len(tran.domain.attributes), 100)
         self.widget.ncomponents = 101  # should not be accesible
         with self.assertRaises(IndexError):
-            self.send_signal(self.widget.Inputs.data, data)
+            self.widget._setup_plot()  # pylint: disable=protected-access
 
     def test_migrate_settings_limits_components(self):
         settings = dict(ncomponents=10)
@@ -84,9 +83,11 @@ def test_variance_shown(self):
         self.send_signal(self.widget.Inputs.data, self.iris)
         self.widget.maxp = 2
         self.widget._setup_plot()
+        self.wait_until_finished()
         var2 = self.widget.variance_covered
         self.widget.ncomponents = 3
         self.widget._update_selection_component_spin()
+        self.wait_until_finished()
         var3 = self.widget.variance_covered
         self.assertGreater(var3, var2)
 
@@ -98,10 +99,11 @@ def test_unique_domain_components(self):
 
     def test_variance_attr(self):
         self.widget.ncomponents = 2
-        self.send_signal(self.widget.Inputs.data, self.iris)
+        self.send_signal(self.widget.Inputs.data, self.iris, wait=5000)
         self.wait_until_stop_blocking()
         self.widget._variance_ratio = np.array([0.5, 0.25, 0.2, 0.05])
         self.widget.commit.now()
+        self.wait_until_finished()
 
         result = self.get_output(self.widget.Outputs.transformed_data)
         pc1, pc2 = result.domain.attributes
@@ -162,8 +164,8 @@ def test_normalize_data(self, prepare_table):
         # Enable checkbox
         self.widget.controls.normalize.setChecked(True)
         self.assertTrue(self.widget.controls.normalize.isChecked())
-        with patch.object(preprocess, "Normalize", wraps=Normalize) as normalize:
-            self.send_signal(self.widget.Inputs.data, data)
+        with patch.object(preprocess.Normalize, "__call__", wraps=lambda x: x) as normalize:
+            self.send_signal(self.widget.Inputs.data, data, wait=5000)
             self.wait_until_stop_blocking()
             self.assertTrue(self.widget.controls.normalize.isEnabled())
             normalize.assert_called_once()
@@ -171,8 +173,8 @@ def test_normalize_data(self, prepare_table):
         # Disable checkbox
         self.widget.controls.normalize.setChecked(False)
         self.assertFalse(self.widget.controls.normalize.isChecked())
-        with patch.object(preprocess, "Normalize", wraps=Normalize) as normalize:
-            self.send_signal(self.widget.Inputs.data, data)
+        with patch.object(preprocess.Normalize, "__call__", wraps=lambda x: x) as normalize:
+            self.send_signal(self.widget.Inputs.data, data, wait=5000)
             self.wait_until_stop_blocking()
             self.assertTrue(self.widget.controls.normalize.isEnabled())
             normalize.assert_not_called()
@@ -185,13 +187,14 @@ def test_normalization_variance(self, prepare_table):
         # Enable normalization
         self.widget.controls.normalize.setChecked(True)
         self.assertTrue(self.widget.normalize)
-        self.send_signal(self.widget.Inputs.data, data)
+        self.send_signal(self.widget.Inputs.data, data, wait=5000)
         self.wait_until_stop_blocking()
         variance_normalized = self.widget.variance_covered
 
         # Disable normalization
         self.widget.controls.normalize.setChecked(False)
         self.assertFalse(self.widget.normalize)
+        self.wait_until_finished()
         self.wait_until_stop_blocking()
         variance_unnormalized = self.widget.variance_covered