my iMac is running very slow

Welcome to Apple Support Communities.

Notes:

• Adobe Flash Player installed Delete Flash. [Instructions]
• No Time Machine backup It is very important to have a backup!
• Delete McAfee. [Instructions: Go to the Applications Folder, and find and Double-click the McAfee Internet Security Uninstaller.]
• Delete the McAfee WebAdvisor Safari Extension.

Other than that, the reason your iMac is very slow is due to the 5400 rpm hard drive. Very slow. If you use an external SSD and run macOS from that, it will make your Mac 10 to 30 times faster for storage. For more info, instructions, and what external SSD to buy, please read: How to Setup and Use an External SSD as y… - Apple Community.

-Jack

You have several factors hurting performance:

1) This is the "educational/institutional" Mac with a crippled dual core process instead of the more robust 4-core processor use din standard models.

2) Entry-level iMac from 20120 to 2019 have badly under-spec hard drives that are demonstrably SLOWER then the drives in entry-level 2011 iMacs.

3) You have only 8GB RAM that, in 2015 models, cannot be upgraded even by the gods of Cupertino; RAM is integral wth the logic board instead of slotted.

4 You installed MacAfee. EVERY commercial anti-virus product ported from Windows a) is useless on Macs, and b) will slow your Mac considerably. And MacAfee has crashed your computer six times in the last 30 days.