Some suggestions here:
http://forum.1cpublishing.eu/showthread.php?t=21398
inlcuding:
Make sure the Anthromorphic setting is off
In Nvidia Control Panel set Maximum prerendered frames to 0
Try reducing screen resolution
Not perfect answers but may be worth trying.
They also ask if the screen graphic of the Fire button responds immediately or if it also lags as it might point to a cause (whatever that might be).
The community may not be able to help but without you telling us your sytem spec there's no real chance.