It would be beneficial to allow speed settings (outline, infill, support, bridging, cooling speed reduction, etc) to be specified at a fixed speed instead of a percentage. This would allow the speeds you want to keep constant, regardless of adjustments to the default print speed, to be set. This could be determined by the presence of a % sign at the end of the speed. To migrate existing settings you could automatically add a % sign to all speed settings where applicable and write a flag or version number to the fff file. I can't see a way around providing backwards compatibility for this change unless you already have some sort of fff file version checking implemented.