set prefetch state default to true only in network mode

default to false in regular fork mode (it introduces an overhead on
every call which might be worse than just fetching storage lazily)

boa/environment.py

@@ -388,7 +388,7 @@ class Env:
     _coverage_enabled = False
     _fast_mode_enabled = False
     _fork_mode = False
-    _fork_try_prefetch_state = True
+    _fork_try_prefetch_state = False
 
     def __init__(self):
         self.chain = _make_chain()
@@ -686,7 +686,7 @@ def execute_code(
             ir_executor=ir_executor,
             contract=contract,
         )
-        if self._fork_mode:
+        if self._fork_mode and self._fork_try_prefetch_state:
             self.vm.state._account_db.try_prefetch_state(msg)
 
         origin = sender  # XXX: consider making this parametrizable

boa/network.py

@@ -77,6 +77,9 @@ class NetworkEnv(Env):
     mutable functions and contract creation via eth_sendRawTransaction.
     """
 
+    # always prefetch state in network mode
+    _fork_try_prefetch_state = True
+
     def __init__(self, rpc_url, accounts=None):
         super().__init__()
 
@@ -190,6 +193,8 @@ def execute_code(
         ir_executor=None,  # maybe just have **kwargs to collect extra kwargs
     ):
         # call execute_code for tracing side effects
+        # note: we could get a perf improvement if we ran this in
+        # the background while waiting on RPC network calls
         computation = super().execute_code(
             to_address=to_address,
             sender=sender,