Yeah I have the same issue, randomly, on separate computers. I have checked for malware but none has been found. Besides it only happens on the USMB, not on any other site.
I had just come from another forum to USMB when this happened, causing me to mistakenly think it was happening only here. But when I exited here and went to a different forum it was happening there, too.
So I shut this computer down and went to a different computer. I logged on to USMB and everything was normal. Then I came back to this computer, logged onto USMB and things were back to normal.
So in my experience the problem was not confined to USMB.
To summarize; the specific symptom was a noticeable slowing of typed input.
When backspacing to erase text the response was extremely slow, taking almost a full second to erase each character.
When normally batch erasing you anchor with the left-click and drag over the target area, which instantly is highlighted as you drag. During the slowed symptoms, dragging over the target area had no immediate effect. But after releasing the left-click anchor the target area highlighted after about two seconds.
These symptoms are identical to the effects of a remote key-logger attack.
Are these the same symptoms you experienced?