Nah, you are overthinking it. You can record how many frames or cycles it takes for the first thing to appear on the screen. Since the emulator is determinist

Nah, you are overthinking it. You can record how many frames or cycles it takes for the first thing to appear on the screen. Since the emulator is deterministic and you are always starting from the same state, it will always take the exact same amount of emulated time (for each version of the emulator). So your progress bar is simply a measure of emulated time. Or take it a step further, and store a savestate from the moment the first thing appears on the screen, and the user's emulation will start from there instead.