Installing Snow Leopard on older iMac

If you are running Linux on this iMac and it is sluggish, then most likely the hard drive is worn out or more likely failing. The other possibility for Linux being slow is not enough memory (usually 4GB is sufficient although I have used Linux on a Mac with just 2GB of RAM). You can check the health of the hard drive by running GSmartControl which should be available in the distribution's software repositories or if you are familiar with the command line you can use the "smartctl" utility which is part of the "smartmontools" utility package which is usually installed by default in many Linux distros. Post the hard drive's health report here using the "Additional Text" icon which looks like a piece of paper.

Another reason Linux may be slow is due to the Desktop Environment you are using in Linux. I know that Gnome3 requires a more powerful system and more memory. KDE or MATE are decent options and only use 400MB when booted to the Desktop with a default install. If you are new to Linux, then Linux Mint or one of the Ubuntu flavors such as Kubuntu or Ubuntu-MATE are good options.

In order to install Snow Leopard you will need to at least create a new empty partition if you want to keep the Linux installation. If you want to keep Linux, then you will need to use one of the Linux utilities to resize the partition so you can create a new empty partition for macOS to use. Once you have a free partition available you can use Disk Utility within the macOS installer to erase the new partition as MacOS Extended (Journaled). If you no longer want to keep Linux, then you will need to partition & format the whole physical drive using the instructions in this article:

https://www.owcdigital.com/assets/support/support-formatting-and-migration/Mac_Formatting_6-10.pdf