Should I format my iMac?
If you are new to Mac, the first thing is prepare:


Make a Time Machine full backup to a drive over USB locally.
See if the Mac boots to recovery - hold Command + R when it starts


Assuming you have both the above, you can wipe and see what a clean Mac looks like. The internet download of the installer is 5 GB or so and the wipe / install should take less than 30 minutes once the download is done.

At that point you can see if you are missing apps and worst case - restore from the backup as if you didn't wipe in the first place - only using time.

You probably don't need to wipe to analyze the performance, but it could help clean up any oddball installations like third party flash / malware / odd browsers and old software you don't need running. A clean start is great to time the things you care about and then see if the system slows down over time.