high CPU usage on pressing any key in Windows

To reproduce:
Open a new document, and open Task Manager. Leave Task Manager somewhere such that you can see CPU usage even when Task Manager isn't the active window.
Press any key, and don't move your mouse cursor.

Note that CPU usage will jump to max (which is 25-30% on my machine). When you move your cursor, CPU usage will drop back to 0%.

There are some differences between the various keys. Modifier keys such as Ctrl and Alt only require a KeyDown to max the CPU. Other keys, such as letters, numbers, and spacebar, require both a KeyDown and KeyUp before the CPU activity will max.

Reproduced on some revision past 0.91pre2 64 bit on Windows 8.1. Also reproduced on 0.48.4 on Windows 8.1.
Not reproduced on 0.48.4 on Ubuntu in VMWare.