The issue with SHDL is not how fast you can mash B, but rather how fast you can press B after C. The amount of time between you pressing c to jump and B to laser needs to be minimized until you can get 2 lasers out in 1 shorthop. Just mash B repeatedly after pressing C, that's the easy part. If you want to be ultra pro you can find the rhythm of the two lasers so you only press B twice, but I have a feeling that would be pretty **** hard to do consistently when mashing works fine.
It definitely takes practice to get it down and to increase the speed of your fingers. Also, as the above post says - button sliding helps to reduce that gap between pressing jump and lasering.
Once you have it down doing it in place, try doing while moving slightly forward/advancing and then slightly backward/retreating. As far as I know it's impossible to do an SHDL out of a full-speed forward shorthop, or at least I've never made it work (For this situation a delayed SHL is better). But small advancing or retreating movements work with SHDL quite nicely.