My configuration is
Window 7 Enterprise, 64 bit
Internet Explorer 11.0.42

Now that I am typing this, everything seems to be working fine...
But now the more I am typing/editing, it slows down to a few characters
per second and now to about 5 chars per second... going decreasing
with each character. Also cursor movement is affected.

Funny. Maybe the editor re-reads the whole text on each character
typed. I should get a fast ASR-33 teletype ;-)

If I try the same in Google Chrome I do not experience this slowdown.
So probably a problem with an Internet Explorer update.