Even when plugin discovery is cached, there's a separate database hit to retrieve the plugins when using the database cache backend.
Several plugins are used on every page request, so this can mean a dozen or more queries every request. This is a performance regresssion from info hooks, where the module_invoke_all() call would be quite cheap (except in certain cases).
Opening this issue to discuss whether we can optimize that - i.e. by using CacheCollector or similar.