Retrieving block plugins with context definitions from cache is expensive, which is noticeable when used with layout builder's FieldBlockDeriver

Problem/Motivation

I have a site that has a fairly large field map. After running some profiling, it looks like deserializing the 'block_plugins' cid from cache_discovery is expensive.

Some screenshots pulled out of the profiling:

This is particularly problematic since this cost is paid on every bootstrap, since retrieving the definitions from cache is all that is required to slow this down.

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes

Release notes snippet