I want to search the file but $findfile too slow and lock up the app. I tried with whilefix.dll but it contains the bug and doing its job perfectly. so any idea?

I would reccomend /filter

use filter with the wildcard option to filter out all matching lines to a temp file, thus shrinking the total number of lines to search for... if only 1 line matches then bullseye...

Also, be aware that $findfile has an internal loop, which hixxy explains in this thread.
imo, this is not obvious from the helpfile.