I sent this in as an email to the staff, but I didn't think to ask if anyone else is having this problem.
I've been using an Intuos since I got this program back when it came out, and while I've been having other technical issues due to it being on Windows 10 (noted elsewhere in the forums), it's worked fine for me, performance-wise.
Today I used a Cintiq for the first time, and it was completely different. You know how the stabilizer works, in that the brush deliberately slows down behind your cursor? Imagine that, but all the time, even when the stabilizer is off, and it gets worse and worse as time passes, to the point where it's moving at a crawl. When the program first starts up, it works fine enough to paint with, but it's still noticeably slower than other programs like Photoshop and SAI, which are spot on and stay that way. But within five minutes, I can barely paint. Sometimes it only takes ten seconds, really depends on the size of the painting, which in this case was just the default size you get when it opens.
I checked my GPU drivers and they're up to date. Processor is 2.5gHz, which isn't the most powerful, but it's been enough to handle Photoshop and this. And as I said before, the Cintiq works just fine with other programs, and even my other tablet worked fine with this one, but something about these two makes a deadly combination, and not in the good way.