What experience of using them are you talking about though? Are you saying when you run a PS1 emulator, for example, you are getting low res and shite fps and you assume that's how it's meant to be?
Some games work better than others, and some consoles are easier to emulate than others. However it sounds like your rig isn't that powerful so it struggles? Because if it's a modern machine and you are struggling to run a ps1 game, then it's likely you have something set up wrong.
As for the resolution thing, you may be confusing the output to the real hard thing, tricking the actual game engine to render at a higher resolution. The former is easy, the latter is really difficult, though not impossible.