I haven't experienced any lag but I don't run photoshop.
Try this for calibration problems:
Reset any existing calibration in Tablet PC settings.
Press Win key, type "Run"
Copy and paste the following into the Run window:
tabcal lincal novalidate XGridPts=10,60,110,360,660,960,1260,1560,1860,1910 YGridPts=10,60,170,330,490,650,810,970,1020,1070
Run the command, allow permissions, go through the calibration. This makes for near perfect pen accuracy, but it's never going to be perfect. Corners will still be messed up, but that's how it works, apparently. More X and Y points can be added to the lists for even more precision. Each number just corresponds to the X or Y coordinate of a pixel on the screen.