add a per-post option

lib/hexo/post.js

@@ -233,6 +233,10 @@ class Post {
     data.slug = slugize((data.slug || data.title).toString(), { transform: config.filename_case });
     data.layout = (data.layout || config.default_layout).toLowerCase();
     data.date = data.date ? moment(data.date) : moment();
+    // Whether to allow async/concurrent rendering of tags within the post.
+    // Enabling it can improve performance for slow async tags, but may produce
+    // wrong results if tags within a post depend on each other.
+    data.async_tags = data.async_tags || false;
 
     return Promise.all([
       // Get the post path
@@ -413,6 +417,9 @@ class Post {
         // If rendering is disabled, do nothing.
         return cacheObj.restoreAllSwigTags(content);
       }
+      if (!data.async_tags) {
+        return tag.render(cacheObj.restoreAllSwigTags(content), data);
+      }
       // We'd like to render tags concurrently, so we split `content`
       // by top-level HTML nodes that have swig tags into `split_content` array
       // (nodes that don't have swig tags don't need to be split).