Extreme lag after Syndra ult?

This has been going on for a while now but call me lazy because I haven't bothered asking around here for help until now. Every time I use Syndra's ult (and hers alone, never happens with anyone else's), I get a few seconds of EXTREME lag/frame-rate decrease. It'll be running smooth at 140-160 fps, then all of a sudden I'm watching other people (and myself) teleport around like mad. Like I said, after a few seconds, it disappears as if nothing ever happened. I sincerely doubt it's my computer lacking power, as I can run league at max settings with 140 fps minimum and it never happens with ANY other champion. Anybody have any ideas on how to fix this?