Add cache to speed up compiling media declarations for stream blocks

As proposed by @pimarc in https://github.com/wagtail/wagtail/issues/3062#issuecomment-586331423

CHANGELOG.txt

@@ -18,6 +18,7 @@ Changelog
  * Allow free tagging to be disabled on custom tag models (Matt Westcott)
  * Allow disabling page preview by setting `preview_modes` to an empty list (Casper Timmers)
  * Add Vidyard to oEmbed provider list (Steve Lyall)
+ * Optimise compiling media definitions for complex StreamBlocks (pimarc)
  * Fix: Added ARIA alert role to live search forms in the admin (Casper Timmers)
  * Fix: Reorder login form elements to match expected tab order (Kjartan Sverrisson)
  * Fix: Re-add 'Close Explorer' button on mobile viewports (Sævar Öfjörð Magnússon)

CONTRIBUTORS.rst

@@ -438,6 +438,7 @@ Contributors
 * dtwm
 * Steve Lyall
 * Lars van de Kerkhof
+* pimarc
 
 Translators
 ===========

docs/releases/2.9.rst

@@ -31,6 +31,7 @@ Other features
  * Allow free tagging to be disabled on custom tag models (Matt Westcott)
  * Allow disabling page preview by setting ``preview_modes`` to an empty list (Casper Timmers)
  * Add Vidyard to oEmbed provider list (Steve Lyall)
+ * Optimise compiling media definitions for complex StreamBlocks (pimarc)
 
 
 Bug fixes

wagtail/core/blocks/base.py

@@ -73,8 +73,20 @@ class Block(metaclass=BaseBlock):
 
     def all_media(self):
         media = forms.Media()
+
+        # In cases where the same block definition appears multiple times within different
+        # container blocks (e.g. a RichTextBlock appearing at the top level of a StreamField as
+        # well as both sides of a StructBlock for producing two-column layouts), we will encounter
+        # identical media declarations. Adding these to the final combined media declaration would
+        # be redundant and add processing time when determining the final media ordering. To avoid
+        # this, we keep a cache of previously-seen declarations and only add unique ones.
+        media_cache = set()
+
         for block in self.all_blocks():
-            media += block.media
+            key = block.media.__repr__()
+            if key not in media_cache:
+                media += block.media
+                media_cache.add(key)
         return media
 
     def all_html_declarations(self):