I just updated to PhotoMechanicPlus R3215 version and tested to add from catalog to collection 3892 files.
This is database level thing (no need to query actual files , just read and write to database or am I misunderstand usage of catalogs) and should be fast
My test took more than hour.
I had filter by : folder path
Selected all of selected files at light table and used right click menu to Add to Collection : xxx
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