League not smooth despite good fps: Tried everything. (Weird issue)

Hey guys, I have a weird issue. I have a pretty good PC (good enough for league: i5 3570K@3,4GHz, GTX 1080, 20Gigs of RAM) but the game isn't "smooth". A friend of mine has the same issue with a super beefy PC (also NvidiaxIntel, but way more powerful CPU) and now I see what he means. It's really hard to describe. Sometimes it feels as clicks/skills don't go through, or like input lag. I can't explain it. Other games are fine for me. We have tested every possible thing there is to test. Different monitors, we basically switched the whole PC of my friend except for the PSU (I have the same issue too, keep that in mind). Graphics card removed, same issue. Windows reinstalled, obviously LoL reinstalled - same issue. Tested a lot of in-game settings as well. We both connect through LAN. Is League maybe still running from CPU graphics for both of us? How can I troubleshoot this? Thanks for any help!
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