I do use KOReader almost exclusively on all my devices
Never had any issue with the metadata search. I have a few gestures to jump to series/tags and other that pops up the main search interface. For a few hundreds or thousands of books it should be pretty snappy (ie: search in less than 0.1s)
Anyhow, the search part of the calibre plugin in KO is undocumented, so if somebody of the current users would want to make it easier for other interested parties feel free to fill the void in
https://github.com/koreader/koreader...etadata-search. Much appreciated