Lots of the time it seems that the limiting factor in FDM 3d printing speed is the filament extrusion rate. This is all done by a gearwheel that shoves the filament into the hot bit to melt it. This has a fundamental limitation though in that it has to heat the filament from the outside in.
You can see people try to get around this with weirdo 3 core melting nonsense and whatnot. Instead of this though I reckon a small magnetron emitting something in the 10s of GHz would be just the trick as it would heat the filament though. I don’t know how much of an improvement this would give, though.