CPU: Ryzen 5 1600 (12nm version)
GPU: Radeon HD 7950
GPU driver version: 22.6.1
Reproduction rate: every game
What overlays are you running (Blitz etc)? None
Did running the game in DX9 mode help? (You can do this via settings/Game/Prefer DX9 Legacy Mode) Yes
Did running with SMT/Hyperthreading turned off help? Yes
don't have the rpd files, since I don't want to play another game with those lags, but here are pictures of my recorded frametimes during three matches, the parts where it lags are clearly visible:
https://abload.de/img/lol_1o4eva.png
Yeah. That looks like it. Thanks for the images.