The link to Perfect6502 is interesting. It says that it supposedly gets speeds of 1/30th of 1MHz - i.e. ~30KHz - on high-end 2025 CPUs. Which is very slow of course, but still ~1000x faster than what people are reporting in this thread.

Of course it's in JS rather than C, but V8 is really good, well within an order of magnitude - not three - of native, at least with well-written code. I profiled the original Visual6502 and at a glance it sure does look like it's losing a ton of performance to rendering (so good call there).

I have to wonder what the limit is. Even Perfect6502 looks like it's all single-threaded and CPU-driven, which really isn't making the most of modern machines. But I don't know if this is a task that could be efficiently split across cores or dispatched to the GPU (and I can't even begin to know whether SIMD might be useful); of course hardware itself is fundamentally parallel, but the synchronization might kill you?