Hm, you're right, JSCL seems abnormally slow on this loop (27.5 sec here). SLip consistently takes 19.3 sec. My CPU is rather beefy (Ryzen 9 HX 370). What's your hardware and browser?

But JSCL is much faster on the fib example, that's why I thought it should be generally faster. After all, it compiles to JS.