I've had this problem recently,
My Wacom Bamboo Pen & Touch tablet wouldn't stop glitching/lagging around the screen whenever I pressed to drag... which was horrible and making it prone to useless.. and it is not my computers' specs fault, although it does run windows /joke ..
So I tried google! since google always has the answer.
Quick Fix: connect the tablet, open the utility "pen and touch" and disable the "press and hold to right-click".
It seems that when you press and hold the pen, it wants to help you out and make a right click. Now, if you have a pen with a couple of buttons which already has right click enabled, you might not need this, thus you can go to Start, and search for "pen and touch", and the utility will pop up! (if you can't find it, and you have installed the drivers, it should be under Control Panel, also called "pen and touch"). Your tablet should be connected to do this btw, I just tried finding it, and the utility was hidden from me since it's not connected... Thank you windows...
So, from here there's something called "press and hold" which corresponds to a mouse right click, and if you select this, press settings, de-check the box, press ok and apply - the pen should stop lagging around when you click and drag. Wonderful.
Also, some suggested that you also disable the "flicks" - direction shortcuts, since this also causes the computer to be wondering what you might want to do every time you put the pen down, so you can also disable this if you're not using it and want it to function even smoother.