Weird FPS drops with Thresh on screen

So, I just played a ranked flex game (Sivir, no skin, in case it matters) and for some reason, every single time my allied Thresh (once again if it matters, they were using the Dark Star skin) was on my screen, my fps would drop from around 70 to around 10. I tried moving my screen around, and this ONLY happened while Thresh was on my screen (and alive, no fps drops while he was dead on screen). As you can guess by the fact that they were my support, it becomes very hard to play when you're lane partner is for some reason making your frames drop. I'll also note that I disabled all of the Star Guardian custom things (the Ahri announcer pack, Star Guardian minions) and it didn't effect anything. I'm used to lag, but I've never seen such a weird case like this before. Additionally, I'm pretty sure I was the only one who experienced these fps drops (although I don't know for sure). Any information on why this might've been happening would be very appreciated, and thanks for reading!