May 2019
It could be anything causing it from a bad 3rd party driver to malfunctioning hardware and how it interfaces to some shoddy program that runs in background or a 3rd party service that's invading spaces it should not.
November 2019
The game has a memory leak, first match will be 90fps [smooth 60 with frame limiter], after several matches my frame-rate will constantly have frame spikes down to 20fps; restarting the game fixes the frame spikes....
I7 6700k 2x16GB DDR4 3200MHZ RTX2080.