I have played with fast terrain on since I started playing League, and have had it on for years now.
I also didn't really use it for the fps boost, it did help, but I used it mostly because it was my preferred way to view the game.
With it gone, I see myself getting bad headaches much faster than I used too.
This is due to my eyes having a slight weakness to bright light, due to an accident that happen to my eyes when I was a little kid. I play with sunglasses on most of the time and will get headaches from playing any game on a computer/tv after awhile.
But the time it took me to get a bad headache before with it on was roughly around 4-5 hours. Now it seems due to the new angle I have to look at, I get a headache at around the 40 minute mark, which greatly decreases my ability to play comfortably.
I won't be quitting league because of this, but I will be playing a lot less(kinda forced), and wish the good people in Riot did a bit more research into WHY fast terrain was used by some people, as some of us used it for reasons other than performance reasons.