When I was using The Bat, which much slower in almost every aspect than eM Client, I have thousand of filters (in fact - “or” rules more than filters, like spam sentences etc.). It takes fast no matter if we’re talking about new emails or re-filtering. Not very fast, but not slow as eM Client.
In eM Client I started to creating own rules (two of them - one that contains from spam addresses and one that contains sentenced to search in subject or body) by adding more and more words/sentences. Problem is that filtering takes looong time. I have 162 addresses and 335 words/phrases and when I re-filter 1000 emails, it takes over minute!
It should be somehow better optimized imo.