@avi12 That's one way of doing it. In the extension I used, I suppose they just stored current speed in some variaible, and whenever+ shortcut was hit, it would increment by 0.25, and - shortcut would decrease by 0.25. Not percentages of actual speed, just a set interval.
I set mine to increase by +0.15 and decrease by 0.10, so I'd get some odd speed like 1.35.
But that would be easy to fix: include a shortcut to put back speed to 1x. So if somehow the user messes things to the point of getting lost, it could switch back to normal with ease.