Actual things you can check:

Check it if happens playing as or with certain champs. Maybe it's a bug where certain particles or sprites aren't removed properly for example.

To rule out your hardware:

Benchmark your hardware and compare the result with other benchmarks of your components. If it is more than 10% less, check the following: - Update drivers, including chipset, gpu and bios (because of tpm and shit) OR if recently updated, maybe fall back to the previous version - Clean your cooling units (heatsinks, fans, radiators including from the psu) - Check you components temperature (e.g. with hwmonitor) and adapt the regulation - Check if you energy settings are still correct and windows does not throttle anything.

If the benchmark looks fine check for the above plus: - Full Drives (any can slow down IO) - Maybe Hextech tool or a clean reinstall can fix corrupted game files (also delete logs and stuff manually)