MongoDB 5.0 local read concern

Hi,

I’m slowly modernising an old application and its database. I’m now up to MongoDB 4.4, next stop is 5.0.

I see in the 5.0 compatibility notes it says:

Starting in MongoDB 5.0, “local” is the default read concern level for read operations against the primary and secondaries.

This may introduce a significant latency increase for count queries that use a filter and for covered queries.

I’ve two questions:

  • Why might there be a significant latency increase - I’d have thought local would be the least expensive read concern?
  • What would be a more appropriate read concern for those kind of count queries?

Thanks in advance!