Significant database request increase following the switch to drupal 9

Drupal Version

Drupal 9

Domain module version

8.x-1.0-beta6

Expected Behavior

The number of request while loading a page after a cache clear should be increased only by the number of existing configuration overrides.

Actual Behavior

On a Drupal 8 project using Redis, after a cache flush, the total number of requests sent to the database to display a page was about 1000 requests of which about half were related to Domain Config and more precisely to DomainConfigOverrider::loadOverrides().

After the switch to Drupal 9, this number of requests after a cache clear increases to 3000 and DomainConfigOverrider::loadOverrides() triggers 2500 requests out of these 3000.

From what I understand, the problem is that in version D8 Domain Config only made a request to retrieve a config override if it actually existed.

After the switch to D9, Domain Config makes a request for all the configurations of the site.

For information I tested version 1.0-beta6 on Drupal 8 and I have 1000 requests and with the same version on Drupal 9 I have 3000 requests.
So certainly something has changed in the core of Drupal between version 8 and version 9 that explains this behavior.

Steps to reproduce

Here the steps on a fresh install without using Redis for a more easy way to reproduce :

• Install Drupal without Domain Config
• Clear all cache
• Display a page
• Using a tool like xhprof see that the number of request is ~1659
• Enable domain config and domain config ui
• Create a domain
• Go to Configuration > System > Basic site settings

and override the configuration for the domain created above

• Clear all cache
• Display a page
• See that the number of requests is now ~2700

I also attached a screenshot of comparison using xhprof.

Proposed resolution

Check that the configuration override exists before to request the database to load it.