I used to have that problem, turning off terminal transparency helped.
Not a fix but at least a workaround. I suspect the video driver because
it was a change in driver that caused it, if I recall correctly.

Quoting Richard Watson (2017-03-08 03:17:18)

> Hello
>
> I've been a long-time xmonad user (over 5 years) on all my machines,
> installing using the Debian package.
>
> I'm currently running Debian jessie kernel 3.16 on my main desktop.
>
> Recently I upgraded Debian on my primary desktop (jessie kernel 3.16).
> I'm running xmonad v0.11,  instead of v0.10 on my other machines.
>
> I notice that when certain GUI application are running (e.g. Chrome,
> vmplayer, but not firefox & thunderbird) my urxvt terminal renders
> *really* slowly, redrawing the window line at a time! I also notice a
> huge X server CPU load (25-50%, as shown by top).
>
> I've looked at X server logs and config and cannot see anything that is
> unusual.
>
> Could there be an xmonad issue here? Has anyone else experienced
> something similar?  Any hints as to how to debug this?
>
> --Richard Watson
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