Performance: Prevent extra Layout Builder code from running when rendering fields in isolation (Views results, FieldBlock, etc)

1. +++ b/core/modules/layout_builder/src/Entity/LayoutBuilderEntityViewDisplay.php
   @@ -239,12 +237,29 @@ protected function contextRepository() {
   +  /**
   +   * Determines if this display is safe for usage by Layout Builder.
   +   *
   +   * @return bool
   +   *   TRUE if this display is safe for use by Layout Builder, FALSE otherwise.
   +   */
   +  protected function isRecursionSafe() {
   

   This doc comment doesn't really match the method name. I think we should change it to something more descriptive. Perhaps "Determines if this display can be rendered without causing infinite recursion"?

2. +++ b/core/modules/layout_builder/tests/src/Kernel/LayoutBuilderEntityViewDisplayTest.php
   @@ -61,4 +61,36 @@ public function testGetRuntimeSections() {
   +    $reflection_method = new \ReflectionMethod($display, 'isRecursionSafe');
   +    $reflection_method->setAccessible(TRUE);
   +    $result = $reflection_method->invoke($display);
   

   This seems sneaky to me. Couldn't this be tested by calling the public isLayoutBuilderEnabled() and buildMultiple() methods instead, to more closely test the way that actual run-time code will interact with the class?

Thanks @tim.plunkett. Let me know if you need me to provide any more profiling data to get to the bottom of this one!

Big improvement (as mentioned in slack). From 25+ seconds to 50 milliseconds

Before:

https://blackfire.io/profiles/62be7e91-f22f-49e5-98bf-a5b746e737ba/graph...()

After:
https://blackfire.io/profiles/11d5cdd0-cbe3-466a-982c-13793cdf3b66/graph...()

+++ b/core/modules/layout_builder/src/Entity/LayoutBuilderEntityViewDisplay.php
@@ -251,12 +250,28 @@ protected function contextRepository() {
+    // Layout Builder can not be enabled for the '_custom' view mode that is
+    // used for on-the-fly rendering of fields in isolation from the entity.
+    if ($this->isCustomMode()) {
+      return $build_list;
+    }

I'm wondering why this code is not using isLayoutBuilderEnabled instead? Why should buildMultiple still run even $this->getThirdPartySetting('layout_builder', 'enabled') is FALSE? I must miss something obvious.

@dawehner: Apparently that's exactly what #2976148: Layout-based entity rendering should delegate to the correct section storage instead of hardcoding to either defaults or overrides removed, a bit confused too but I also didn't read through that issue.

Patch looks good & makes sense. Confirmed to solve this major perf issue for me. In my case I was not using field blocks that was causing the problem, but instead I was having issue with views output that was rendering the results using the _custom view mode (AKA rendered individual field output instead of rendered entity).

Just wanted to call out #10 again:

From 25+ seconds to 50 milliseconds

Committed 44ffb27 and pushed to 8.8.x, cherry-picked to 8.7.x. Thanks!