FPS dips when I play Syndra

I'm not sure if this is common to other players (plus I play on laptop), but whenever I play Syndra my usual 90 fps just starts dipping at 65 -- 75 FPS. And that's only during the first 25 minutes of the game. After 30 minutes or longer, the game's performance starts to degrade a bit in general. In Syndra's case, playing that champ after 30 minutes tanks my fps to 45 - 60. I'm not sure if this happens to anyone else, but so far Syndra has been the only champion I've noticed with this issue. Doesnt even matter which skin I use.