it all really depends on the difference in hardware, like m0skit0 said, the psp had a similar cpu to the ps1 (both mips), that is why it only took 333mhz the psp had to fully emulate the ps1 and it's also why it takes 1ghz of ARM power to emulate the ps1.
another thing it also depends on is the technical specs behind the emulated system.
For example, the psp was easier to code for than the ps2, which is why emulating a ps2 is harder, this is also the reason why the dreamcast didn't have saturn compatibility.
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