iMac 24”

Have you installed and run any "cleaning", "optimizing", "speed-up", anti-virus or VPN apps on your Mac? If so that may be cause for the slowdown. Uninstall any according to the developer's instructions.

Depends on what games she wants to play. What are the requirements for those games. Are they Mac only games? Does she play other games using some other apps or software? Such as virtualization or Bootcamp?

It won't run very old games that may work on the old iMac or Windows games using Bootcamp or Virtualization.

It also won't run the latest top of the line games all that well.

If you tell us the types of games she plays we may be able to give you a more specific answer.

Are these online games?

In any case get a 24" iMac with the 16 GB of RAM, not the 8 GB.

A new iMac would ship with Sonoma or Ventura. So it would depend on whether the games are compatible.