I've implemented more or less the same interpreter using computed goto and various variations on dispatch loops; and this is the fastest solution I've managed to come up with, and the nicest code to work with. Still too many parameters, my point is that I have plenty of experience pointing in that direction which is better than nothing. It's not a mystery to me. Using computed goto will involve some kind of lookup to get the relevant labels, I'm using pointers to actual instructions as jump targets. And since I'm longjmp'ing out, which means I don't need a loop condition; the loop is reduced to a single regular goto.
Like I said, better than nothing. There is no such thing as perfection in this world. I've spent quite some time [0] micro benchmarking different features to get a fair comparison.