Geometry Dash has a move speed bug. Geometry Dash move speed is changing automatically, and I think it is a bug. Can I fix this bug?
What you are experiencing is not a bug and can be easily fixed. Go to the Options and disable 'Smooth Fix' (if you are playing the Steam version, it should be under 'Advanced Video Options').
This setting slows down the game instead of dropping framerate. I personally also like disabling it and playing with lower framerate, since Smooth Fix adds inconsistency to timing and offsets the soundtrack.
If im right, you are telling about player x-speed change, while playing long levels? If so, that's impossible to fix right now, cuz that's the bug in the game/code itself. The cause of this is unconsistency in sum of numbers. Example: 1+1=2 is wrong, but 1+1=2.0000000001612 is right, and by progressing in the level further, it's unconsistency grows bigger and bigger. If you wan't to know more about this, just watch this video "GEOMETRY DASH MECHANICS 1/2" it's in russian, but has english subtitles, so no worries