How long is the list of games that zsnes doesn't have 100% accuracy? It had reasonable performance in the Pentium 3 era. We're talking about emulating a 4MHz processor.
Notably, Yoshi's Island glitches out to the point of unplayable-ness in around half a dozen levels under ZSNES, the first one being 'Touch Fuzzy Get Dizzy'. This is the most high profile example.
bsnes is the only emulator that lets you play yoshi's island properly, which happens to be many people's favourite game on the platform.
The problem is down to some games exploiting timing tricks in the graphics subsystem to achieve transparency-like effects and so on. All the emulators apart from bsnes compute the graphics line-by-line, which breaks these tricks. bsnes draws it's output pixel-by-pixel, like a real snes, which is much more computationally intensive.
"The reason is because while a scanline renderer runs at a mere ~15.75khz, a clock-based renderer would run at ~10.75mhz. That may not seem like a lot, but when you are writing an emulator that claims absolutely flawless interprocess communication, that means synchronizing two separate processors over ten million times a second."[1]
All that context-switching makes your processor mad.
The main reason for bsnes is homebrew snes developers' need of a bit-for-bit accurate snes emulator. These people need an emulator that will help them test code they are planning to flash to a cart and run on a real snes. ZSNES and other emulators designed to simply run commercially produced games on their PCs are riddled with hacks and shortcuts, this is what allows them to run fast on low spec machines. However this means that they allow many, many things that a real snes won't, and are useless for development targeted at real hardware.
For more details, see byuu's many rants on the subject, the first of which is here: [1] http://byuu.org/articles/emulation-2/ (Bear in mind it was written ~3 years ago, a lot of the problems he mentions he has since overcome, the aforementioned per-pixel video rendering for example. He discusses this in later essays.)
Strange, I love Yoshi's Island and have beat it many times and have never noticed a problem (at least, haven't noticed it to the point that I would remember it. If it was unplayable, I would've noticed). I'll have to load it up again sometime and see if I see a problem.
Years ago, earlier versions of zsnes did not exhibit this bug. However it used to simply fail to render lots of stuff. What was left behind would still look fine, so you wouldn't notice. Think parallax background layers, sprite effects.
