I could figure out what makes MC work hard for me. It's one of my sorting options.
I have an expression field like this:
FormatNumber( IfElse( IsEmpty( [BPM StFeder] ), FormatNumber( [BPM calculated], 1), IsRange( Math( abs( replace( [BPM calculated], /,, .) - [bpm StFeder])), 0-1.9), FormatNumber( [BPM calculated], 1), 1, [BPM StFeder]), 1)&datatype=[number]
It compares the calculated bpm field with my own counted bpm. I let MC sort ~42000 files based on this field. If I remove the sorting option everything is pretty much snappy again.