Maybe that's related to FS Protection. I tried both the stable and beta version of F-Secure like more than 6 months ago (long time by now) and with FS Protection had delay on almost everything but for the stable version there was no delay when running signed apps. The stable version only had significant delay with relatively high CPU & disk usage running unsigned apps after every signature update.
So, wondering if you have tried any other third-party AVs recently, excluding FS Protection? Like Kaspersky for example (free or paid) which IMO has absolutely the best cache management, so things run very fast if it was already ran before once at least/a full system scan was performed.