When i render video in After Effects, it uses only CPU. While rendering CPU rises from 4% to 70% but GPU stands at 4%. I found some solutions: changed File > Project Settings > Video Rendering and Effects and put to use Mercury GPU Acceleration instead of Mercury Software Only. However it does not help. Any ideas ? I have installed all drivers After Effects sees my GPU.
You have a wrong understanding of how this works. Very little stuff in AE is actually GPU accelerated and for final rendering stuff liek accelerated previews wouldn't even come into play. Likewise, effects simply may fall back to CPU rendering due to how the project is constructed and exceeds GPU resources and/ or a combination of specific features prevents GPU usage. And finally GPU rendering itself is limited to a few formats like MP4/ H.264 or some flavors of ProRes. So what you are seeing is likely perfectly normal within AE#s rendering logic. If you want more specific advise, you have to provide more detailed information about actual render settings, contents of the project and so on. there's always a chance soem things can be optimized, but don't expect anything miraculous.