About GPU Usage when playing League of Legends

Sorry for my bad english, I would like to know anyone meet problem when playing League of Legends?
Everytime start a new game, I can get 400-500 FPS, but after 20 minutes, my FPS start dropping until 80-90 FPS.

I using MSI AfterBurner to monitor the GPU Usage, its really weird.
When 400-500 FPS, GPU Usage have 60%. When 80-90FPS. GPU Usage below 15%.

How to make GPU Usage higher? Hope anyone can help, keep lagging at late game :(
I have update latest drivers.