Yep exactly. Even the SNES emulator lags if you turn frame-skipping off. Also, my impression was that emulators typically do all the work in the CPU... Is having a better GPU really an advantage?
Almost always. If the code is emulated to utilize whatever specific capabilities a graphics card has then it will take away from the workload of the processor.
The less rendering of images the processor has to do, the faster it can deliver or refresh.. So think of it this way. If an emulator was coded to work specifically with a certain mobile chipset, it could tell the gpu do render all the images, render the wireframes, and do all the shading on a 3d game. and have the processor just load the files into memory.. Giving you a smooth 50-60FPS.
If the GPU sucks or isnt being optimized you have to put all those chores into the processor alone, which in turn will cause studdering, lag and not as good of a gaming experience.
THink of PC's works the same way just on a different level. Games that come out are designed for a certain GFX chipset or "works best with" Nvidia or Raedon graphics, or require a minimum to operate properly.
It all depends on how the program was written when it comes to emuation, developers have ALOT of bases to cover when writing code. They have to combine compatibility with performance and sometimes you cant have both. ( or you wait for the next generation of hardware to come out and simply "overpower it"