My computers that have updated to Windows 8.1, and therefor IE 11, are all showing this problem: as I type text into the editor in Xenforo, random keystrokes are just ignored and response is sluggish. I tested IE in "no addons mode" and there was no change, so that's not it.
BTW I had the exact same problem here when I was posting my original message, but right now no keypresses are being lost. Since it was happening here, it's not likely due to something unique to my install, especially since I am not using a custom style...
I can't reproduce this issue with IE11 on Win7. I'm still going to check it on Win8.1 though unfortunately it may be something down to just slow performance. I'm not sure much can really be tweaked if that's the issue.
Testing testing testing testing testing the typing here seems to be working fast/fine for me at this moment and that's odd as I still get this problem all the time on my own system.
I was getting it here and on my own system and now I am not seeing it here as I type this... so I wonder what changed.
My computer system is running Windows 8.1 and IE11 and the machine is very fast. I'm not too worried about this as I can work around it and I assume it will probably go away all by itself as something causing it changes at some point.