Yes, I've verified it works with clist_modern. However, even the other setting (when "use improved filtering in contact list" is checked/on) doesn't introduce a freezing delay. Unfortunately, I don't like clist_modern because it eats much more vertical space so I have to scroll my list all the time, which is uncomfortable. Besides, it appears to be quite a bit slower on my old machine.
So the problem is with clist_nicer. While using clist_nicer, I tried incorporating the (unused) setting "use improved filtering in contact list" into Miranda via db-editor++, (by the way, it's the binary (byte) value /Settings/CLC/FilterSearch) and it doesn't have any effect either set to 0 or 1 - as expected. Miranda still freezes when typing a search straight into the contact list.
However, while experimenting I noticed that soon after switching from clist_modern back to clist_nicer, there is no freezing until the next Miranda restart. So I think the glitch might be within Miranda initialization code itself, and clist_modern in some way resets/cleans the problem, while clist_nicer does not. Just a hint for the developers. Do you think I should post a notice somewhere else in these forums?