ITs not just you.
I use no outside program (No blitz, no discord, etc) and i have the same issue.
Lots of people are having the same issue. But riot don't care apparently.
I've clean-reinstalled league multiple times, and gone through complete format of my computer trying to fix this issue.
I give up tho, cuz after all that trouble i went thorugh, severe fps drop is still happening.
This clearly happened after the recent patch, however riot don't think that. they haven't even posted anything about this.
Time to play something else because they will not fix this.