my big lags also take a few seconds to go away, I just have micro lags here and there and they are, trust me, really short lived like 1/3 of a second. The way I notice it? if u connect a bluetooth headphone u can hear these micro lags affecting even audio. It's hard to explain I know. Think this way: let's say u got music playin via Bluetooth and meanwhile u open a safari tab or simply switch from music app to Safari, u can see a super fast hiccup on screen synced with hiccup on audio. Strangely thin not happens with wired audio. Believe me, this is driving me mad. I'm still incline to believe this is a GPU driver related stuff. The fact that everything wrong goes away putting system to sleep and waking it up is the final shot at my psych integrity
Many report of various problem are present even on Apple Support website and other forums. People are incline to think on unoptimized Navi / Vega 20 drivers fault 'cause all kind of problems similar and even more badass are reported for 16" MBP (Navi) and MacPro's Pro VegaII (Vega 20) and WX 5700XT (Navi). I think there's nothing more to try, I just have to wait and see if driver maturing will solve these. Thanks for your support and patience.
EDIT: It's worth noting that prior to 10.15.3 everything worked smooth and I use the same EFI. It's also worth noting that the FaceTime/Zoom/Meet etc. macOS problem which affected a lot of people and found place even on big tech website, was later described as an issue with recent drivers and videochat apps need for video encoding (so GPU usage here). Cannot be a coincidence that I got problems since 15.3 then, and right now again my FaceTime was unusable then and now it is, but it's still slow to open and camera fps is rubbish.