I have observed significantly reduced performance in GUI in Mathematica v10.4. This is especially visible in autocomplete feature delays. This is very evident when I turn some power saving features which throttle my cpu to around 1.2 GHz. But even in the full power processor mode, I can notice that the writing is not as fluent. I compared it with Mathematica v9 on the same computer (running as a vm) and the the difference is huge. When the CPU is in downthrottled mode, the v9 keeps writing fluently, the autocomplete is immediate with no apparent delay, v10.4 can take a second for autocomplete window to appear, making any work almost impossible with autocomplete on. When I use the full CPU power, the writing is still not as fluent as in v9. This was tested on Windows 7 x64 I compared it to v10.4 on Linux on the same machine and there, I cannot tell the difference between v10.4 on Linux and v9 on Windows, it is fast as it should be
So there seems to be an issue in 10.4 on Windows (maybe even 10.3 but I cannot verify that, but I remember 10.0 was quite fluent or maybe I did not notice).
Can someone confirm the issue? (remember, if you have a strong CPU, you may not notice it immediately without direct comparison with v9 or without having to enable some powersaving on your cpu)