It can't be other processes, the most the CPU ever gets up to while I'm waving the pen around is about 1%. It's 99% idle even while it's reading the pen continuously.
They X and Y position of the pen is probably two bytes apiece plus another two bytes of pressure value. Reading and writing 6 bytes to RAM... that can't possibly be a RAM speed problem.
The image of the cursor, if it's even 16 x 16 pixels would only be 768 bytes of display ram to be moving around. That can't be a burden either.
If those were actually pushing the limits of the computer then we'd expect the computer to be freezing every time the pen moved but it doesn't.