Use cacheBackend->setMultiple in ContentEntityStorageBase::setPersistentCache

Problem/Motivation

While doing a performance test, I loaded 1000 entities using {EntityType}::loadMultiple, and the execution was really slow, 15 secs. After some debugging, I found that the slowness is because of ContentEntityStorageBase::setPersistentCache, it contributed roughly 14 secs to the load time.

The code goes through each loaded entities and sets the cache for them one-by-one.
If the code collects all the items first and then call the cache backend only once (setMultiple), the cache setting time decreases below 1 sec and the overall load time of entities to 1.5 secs.

The specific call stack:

1. EntityBase::loadMultiple
2. EntityStorageBase::loadMultiple
3. SqlContentEntityStorage::doLoadMultiple
4. SqlContentEntityStorage::setPersistentCache
5. ContentEntityStorage::setPersistentCache
6. DatabaseBackend::set

Steps to reproduce

• Use {EntityType}::loadMultiple on high number of entities (say, 1000)
• Measure times at the above mentioned calls before applying the patch, e.g.:
  

  $start = microtime(true);
  // the function call
  kint(microtime(true) - $start);
  

• Apply the patch, measure again

Note: My installation is Drupal 8.9.2, I tested with that, but created the patch based on 9.0.x (dev). The same patch applies to 8.9.2 as well.

Proposed resolution

Collect all items to be cached first in an array, then call {cache backend}->setMultiple.

Remaining tasks

• Review the attached patch.
• As the items to be cached are first collected in an array, the memory consumption can slighty increase. Should not be a big issue, but please review this and comment on it.

User interface changes

-

API changes

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Data model changes

-

Release notes snippet

Later, first review the proposal and check if it causes any regression.