I concur,
@levi. AFAIK, what means trouble in emulation is not primarily clockspeed nor is it instruction sets (most platforms today are more capable in terms of hardware acceleration, or emulate quickly enough for the latter). If it was that simple, I guess your high-end smartphone could have a go at X360/PS3 emulation.
What matters most is architecture complexity :
For the best accuracy in emulation, you must synchronize the emulation of the different chips and time them well together, so that results come in the same order as they would on the real platform. This means constantly checking and so introduces massive overhead.
As an example, current emulators of choice like higan (formerly bsnes when it was dedicated to Super Nintendo) are cycle-accurate. This means you don't only execute the program's instructions, but model how the different parts of the machine behave together. If the CPU takes "this long" to retrieve data from RAM, thus slowing down the entire program, you must take it into account. While it can lead to dropped frames and input lag, it is truer to the actual console.
In the SNES case, the master clock is 21.47727 MHz for the NTSC version, while the CPU runs at only 3.58 MHz. Each chip is not on the same clockrate, so anything can happen between CPU cycles. And the SNES is known for having many hardware features, including graphics manipulation
during frame rendering. If your simulation is not tight-knit, it can easily lead to missing or garbage graphics.
The more obscure or difficult hardware features are seldom used by most programmers, but well-thought and heavily optimised code can lead to this "I didn't know it could be done !" moment we all love, like
Air Strike Patrol adding a shadow in the middle of the scanline being rendered ! Also you'll note that the SNES Mini fails to deliver on these features as most games play "well enough" and it would need much beefier hardware. Plus the original SNES used in the video is a 1CHIP revision, so even it is not displaying the effect in the same way.
So, you can understand why PS1 and later N64 games have been playable on PC when they were still on store shelves, and why the Saturn with its two CPUs and two GPUs had to wait years before an emulator showed more than just a few basic demos.
Now, imagine getting the monster that is the multi-threaded PlayStation 3 running more than decently, but soothe your nerves knowing these days games aren't as complex from a "hardware juggling" standpoint. Few of them aren't at least a little platform-agnostic and even fewer make use of undocumented hardware features. With current generations being based on generic technology, it's more a matter of figuring out the APIs called by the software than
stitching textures together.
All of this explains why a Pong emulator simulating every single transistor can make your computer run to a crawl, while N64 games are at least "somewhat playable" since UltraHLE in 1999. While ZSNES and the like are classics and would be perfectly fine with most games, don't trust them to be a perfect representation of the 1990 experience. Have respect for the groundwork leading to current software, but please use the better modern alternatives when you can... or just do as I did and get the real deal and fall into madness trying to figure out how to plug it in today's TVs