In https://github.com/10gen/mongo/blob/master/src/mongo/db/query/sbe_stage_builder.cpp#L128-L139  we end up registering slots for

$$USER_ROLES

$$NOW

$$SEARCH_META

regardless of whether the query uses these variables or not. These slots aren't free, so if we could skip some/all when they aren't used, we'd win some perf back.

Likewise setUserRoles() is called whether we use $$USER_ROLES or not, and that also is not free, and would be great to only run if needed.