Slow mopeds can be ones with leaky seals and gaskets.
They suck in too much air, which makes the fuel/air mix lean.
That robs power and increases heat. So much that it can cause the piston to expand hard against the bore. It will wipe out lubrication and exponentially increase heat due to friction. The piston will melt and get stuck.
Seizing a piston is a bad way to start your moped adventure. Not trying to scare you, just want to make it a first point of caution before to go blasting around on your new machine.
You might have no leaks whatsoever. Could be slow for a dozen other reasons. But, make sure you learn how to check that and rule it out.