@FlameDuck: I don't think that's his problem. I get a mouse lag as well, but it is only a fraction of a second. Long enough to be noticeable, but not an entire second long. Unless his movement requires precision movement, I'd doubt he'd really notice the lag at all.
eta: This will show you the difference between where the mouse pointer is and where the pointer is being rendered.
Graphics 800,600,32 Local oldmousex:Int Local oldmousey:Int Local mouseflag:Int = False While Not KeyHit(KEY_ESCAPE) Cls If MouseDown(1) If Mouseflag Local newx = MouseX() Local newy = MouseY() DrawLine oldmousex,oldmousey,oldmousex,newy DrawLine oldmousex,oldmousey,newx,oldmousey DrawLine oldmousex,newy,newx,newy DrawLine newx,oldmousey,newx,newy Else mouseflag = True oldmousex = MouseX() oldmousey = MouseY() End If Else mouseflag = False End If Flip Wend
eta: This will show you the difference between where the mouse pointer is and where the pointer is being rendered.
Graphics 800,600,32 While Not KeyHit(KEY_ESCAPE) Cls DrawRect MouseX()-10,MouseY()-10,20,20 Flip Wend