X using ~ 35% of a CPU core

[Celejar <celejar@gmail.com>]

Hi,

On my ThinkPad T61 (Core 2 Duo T7300 @ 2.00GHZ) running Debian stable
(8.2) with Xfce, the X process (/usr/bin/X) has recently begun to
consume some 35% (seems to vary between 33-40 %) for several minutes at
a time, with the system becoming unpleasantly unresponsive during this
time. It seems to mostly happen after resuming from suspend-to-ram, and
usually clears up after a few minutes with the system performance
returning to normal. I run mostly straightforward stable, albeit with
self-compiled kernels from upstream (vanilla). I first noticed the
problem on 4.1.13 and am now seeing it under 4.2.6. I don't think I've
seen this behavior in the past, and I'm not sure what has triggered it.
I see it even when not running much of a load under X at all, e.g.,
just recently, when all I had running was Xfce Terminal, Sylpheed, and
one (simple, small) OpenOffice writer document. Any idea what this is,
or how I can fix / troubleshoot?

Thanks,
Celejar