Vind - my only guess is that your computer has a dual core CPU? Course othewise it should work well. Does the lag happen all the time, or as it was with Nimiria - only when you select lots of units?
Nimiria - our programer suggested you try the following:
-Try setting your windows resolution down, and see if that changes anything
-In the Windows Accessibility (or display setting) - there is an option to set Icons to small, large and medium - make sure it`a set to Medium
-If you have made changes to any settings in your video card control panel (NVIDIA Control Panel inside Windows Control Panel) - try defaulting the settings
-An last thing to try - get the latest driver for NVIDIA GeForce GTX, your driver is only 3 months old, but perhaps there is a newer one.