This may simply be an issue with my particular motherboard (Gigabyte Z77-UD5), but would be interested if others could give it a try.
When XHCI is enabled I notice a large increase in mouse lag. The mouse becomes floaty as if vsync has been enabled. Simply disabling it in bios (you lose USB3) has greatly increased mouse responsiveness.
XHCI enabled - floaty mouse, in-game sensitivity 3 to compensate
XHCI disabled - responsive mouse, in-game sensitivity 4 where it used to be on an old USB2 machine
I used a little flash game to test it out. Decided that I noticed it enough to just leave XHCI off. If I ever get something that requires usb 3.0 I can just enable it.
I was thinking that the Usb2 ports were on a separate controller considering you can turn it of in the bios.
It's kind of funny how there are a lot of factors that can introduce lag but not a good way to test it.
So I guess it's the same as V-sync, flip-queue, scaling and what have you. If you think that it lags just turn it off.
My mouse acts funny on USB3, I stick to USB2 for everything that is not specifically a USB3 device, like my external HDD enclosure.
Maybe it's different for other mobos, but on mine the USB3 ports are on a separate controller from the other USB ports, so I can still utilize USB3 w/o negatively effecting the stuff plugged into the USB2 ports.
I was under the impression that it would be fully backward compatible. Just like how I didn't notice anything when usb 2.0 was new.
But now I got an external soundcard that pops on the 3.0 ports and I have to worry about input lag.
It's kind of weird how this mobo has only 2 regular 2.0 ports and no ps/2 port. So you have no slots left by just hooking up your keyboard and mouse.