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Because it only works that way for a small amount of people. Most people actually get a speed up with canvas graphics acceleration turned on, but because they don't have to change it, there's no questions asked about turning it on.

The problem is really caused by bad graphics drivers. We have sort of mitigated this by implementing angle support which lets Krita use the directX driver, and Krita tries to select it for the graphics cards we know are the most affected(and we really recommend trying Angle first), but we cannot figure out mechanically which combination of options works best for most people, so hence the question gets asked, and back in the day when we had no Angle support, we recommended turning it off altogether, which is something people just repeat when trying to help one another. Turning off graphics acceleration loses a lot of navigation speed, so it just isn't a good solution.

Plus, there are quite a few features not available if it's turned off.