Servo has a "webrender" backend that is GPU-accelerated. While it isn't nearly at feature parity with the established rendering engines, it is an order of magnitude faster at rendering the features that it currently has implemented.
It won't replace your current browser for some years, but I see no reason why it couldn't serve as the engine for something like Electron.
Sure but it's still going to be slower than a native GUI library designed for GPU acceleration because you still need to implement CSS layout and DOM which are suboptimal by design. Maybe by the time Servo lands to mainstream it will be fast enough that it won't matter - but right now that's just not true - DOM is still an inferior platform for GUI because the performance sucks and that's not going to change any time soon - Servo isn't even close to complete yet.
Not necessarily. For instance I believe Nyaovim actually renders to <canvas>. So there is very little dom. I'm sure there is some overhead, but I don't see any reason why the performance can't get on par, if not close to native application performance.
It won't replace your current browser for some years, but I see no reason why it couldn't serve as the engine for something like Electron.