The BEST speed is HAND speed. However,I think that variable speed is an excellent way to go. I took an old die filer(a machine that holds files,running them STRAIGHT up and down), and made a variable speed jig saw out of it. Which I've never used myself. Loaned it to another guy who had some marquetry to make.

I have a variable speed Hegner. I don't like these new walking beam jigsaws that seem to be the only ones made now,including the Hegner. Their blades move in an arc,and also go backwards ad forwards as they saw,due to their arc movement. I'd rather have an OLD jigsaw like the old Deltas,or other saws from the 50's and 60's that do go straight up and down.

I also find the blade changing on the Hegner to be a big bother. Since I have little need for jigsaws,I haven't gotten around to making better,more convenient blade holders.

On the old Deltas,you just had the blade sticking straight up,and brought the spring loaded overhead down,tightening the blade with a thumb screw.

I got the Hegner very cheap at a sale where they broke up a shop. That is why I have one at all.