Again, we'll say it once again : it has nothing to do with Mhz... By example, even a SNES with close to perfect emulation needs a 3GHZ X86 computer

In our case we'll just be glad enough with HLE and a dynamic recompiler, which will work for 95% of the titles even with unprecise emulation. BUT, the arm11 in the 3DS is really slow and the memory is really low.
Even the raspberry pi struggled for a long time to have "decent" PSX emulation, and let's face it, the core itself is more powerful and had 4 times to 8 times more memory.
The only positive side would be the PICA200 but we barely know nothing about it. We have roughly a first attempt to implement opengl on it (i looked on other topics..., i'm already impressed by this breakthrough)
Moreover I never said it would be impossible. You could potentially emulate any type of console with any type of hardware: but you'll end up to have <1 fps. You really need a powerfull hardware and tricks (or skills) to gain execution time and get close to the original framerate. I think we could have PS1 emulator on 3DS, but the emulation quality, even with best effort put on it, would look like garbage.
I'm open if someone more clever would prove us I'm wrong, but for now, I'm waiting about proofs and facts
EDIT : example on pandora clocked at 900Mhz, don't even think about 266 Mhz ARM 11 in 3DS :