[VorpalWay]

I'm not sure if a 3.5 will be as fast as a 3.9. You could enable printers you are interested in in the slicer and slice some typical models of yours.

Because of things like acceleration and flow limits the scaling isn't as simple as a constant factor but also depends on the geometry of the part (sharp corners lead to slowing down) and material (check max volumetric flow in your slicer, will likely be set way too low for your current filament profiles, especially if you switch to a high flow nozzle on the nextruder).