First of all, welcome to the forums Katjing!
About the problem -
Why did you do that? Were you suspecting the wireless one was conflicting with the mx518? And how is the performance with the wireless one if you use it alone?
Originally Posted by Katjing
Assuming the mx518 is usb mouse, please post outputs of (while it is plugged) -
And as soon as it starts lagging, enter (in a terminal)-
and post back their outputs.
dmesg | tail -20
cat /var/log/syslog | tail -20
Don't forget to wrap the outputs in code tags while posting them (see the link in my signature for an example)