If you means something that searches for files within any directory, I think the best so far I've used was Everything ( https://www.voidtools.com/ ). Lightweight, fast, and really simple UI. I'm still looking for a Linux equivalent (I'm currently using a shell wrapper for `ag --depth 0 -g `.)
It may seem "lightweight" in a modern desktop machine, but I believe the way it works is: it keeps the index of your entire filesystem in RAM at all times. And it continuously monitors that filesystem to keep the index updated. It's kind of brute-force, really. I can think of some ways that could get out of hand and not seem so lightweight.
Regardless, the gap between what Everything can do and what the built-in File Explorer search can do is an embarrassing canyon, and I can't believe MS hasn't even fixed the built-in search enough to make it function correctly let alone quickly at this late stage.