Hello!
The maximum frame rate seems not to be limited by default: as soon as I turn off vsync, the cpu usage of the client goes from 20% to 100% (even when I'm still in the login screen).
Furthermore, this seems to have no impact on how my graphics card is performing, so my guess would be that whatever is hogging the cpu isn't directly related to rendering but is just done once every frame no matter what - can that be right?