I think that this is the same issue with criterion groups that I posted about here.
My main database now has just over 57,000 people in it. To test the issue I created two groups, a criterion group (has any fact with date < 1900) containing just under 34,000 people and a freeform group containing ancestors and their descendants going back 10 generations and then forward 10 generations (containing roughly 43,500 people).
The larger freeform group takes about 10 seconds to load in the index on the people screen and the smaller criterion group takes about 25 seconds. There is the same delay when you move from other screens to the people screen or move from for example the groups option back to the index option or when you remove a more detailed filter (eg typing ‘Watson’ into the search box and then clearing it) on the index.
In each case, RM9 seems to re-apply the filter, and it each case it takes longer to do this for a criterion group than for a freeform group with more members, even though it is not supposed to do any calculations on the fly for the criteria.
I am running the 64 bit version of RM9 on Windows 11 using a fast, modern laptop with lots of cores etc, 16gb of ram and a fast ssd which is not being synched anywhere else.