I'm using Word 2013 on a Windows 8.1 machine, but I had this same problem on a Windows 7 machine.
When I try to add a comment to a document using the track changes feature, typing in the revisions/comment pane becomes so slow as to be almost unusable.
Typing in the rest of the document is fine/normal. It's only when I'm trying to type (or edit) a comment that the typing slows way, way, way down.
I did a Google search and noticed that this was a big problem with Word 2007, and a lot of people suggested fixes. Not so with Word 2013, as far as I can tell. (That is, I can't find a lot of other people having this problem in Word 2013.)
Any advice would be welcome. Thanks!