Latency spikes caused by Xfce?

Hi,
as i am new here in the forum i will begin with a short story about myself. I am using linuxcnc for more than a year now. The latency numbers my old computer gave me weren't that good and so i recently got obsessed by the idea to get sub 4k latency. I spent hours reading posts here and comparing the machines listed on the latency-test wiki-page.

I saw that the asus p5b deluxe mainboard has very good results, but it has no parallel port (i am software stepping).
So i concluded that the ga-p35-ds4 is the best fit for me, as it also has a parallel port. I bought the board, and now i spent the whole weekend playing around with kernel parameters, irq affinity and so on.

Ok, enough background about me.
What i discovered today is, that the latency-test gives me way worse results with Xfce than with i3 as windowmanager.
With xfce i get irregular spikes of 17k and sometimes even higher.
I was able to reproduce this by opening the "root-terminal" from the menu. This root-terminal asks you for your password on startup. While doing so, it pulls of a fancy graphical effect, that dims your screen. And at that moment, the huge latency spike appears.
So i thought, it has something to do with this graphical effect, i figured that it might help to use a very simple windowmanager.
Thats how i got the idea to try i3.
And the result of this long story is:
i3 windowmanager gives me a solid latency of about 7k while running 5x glxgears, 1080p movie in vlc and surfing the web with firefox for almost 4 hours.

Has anyone an idea about why there is this difference between xfce and i3?

Sounds like some more experimentation is due. Have you tried any other desktops? (I'm not a big fan of xfce to start with.)

I haven't tried any other desktopmanagers yet. I am planing to give lxde and openbox a try. I can do that on friday.

I also ordered the asus p5b deluxe with a parallel PCIe card. I hope i can reproduce the 3500ns latency results from the wiki page.

@tommylight: There are several optiplex on the wiki list but they are nowhere near 1800ns. More like 18000ns