I don't use Gmail on the PC much - I have it for my Android, and use the app. However, I've just logged in on my fairly modest Win 8.1 machine, and my moderately fast typing is accepted with no delays whatsoever in both the new message and the (apparently) new feature reply box. I even just pressed random keys as fast as I could with multiple fingers, and there was no obvious delay. Browser is Firefox 34. I would suggest therefore that it is something running on your machine which is causing the issue rather than Gmail - is your AV somehow scanning as you type?