Hmm ... why not (also) debug the other way round. Try to do things which you suspect should make a currently working browser (MiniBrowser) stop working. Like disable JIT. Run on slow emulator. Or single core machine.

Site on MiniBrowser still worked after 'export JavaScriptCoreUseJIT="0"'

Sometimes info you get from debugging can be deceiving. If it has to do with timing, maybe it's not the timing itself, but timing code implementation/precision.