Imac 21.5 late 2015 Geekbench score VERY low

Yep, I had that very same problem. I didn't care what the score was, but performance was dismal.

I fixed my problem with an external SSD. By the numbers there, the internal HDD read at 90 MBps; the SSD at 450 MBps. A five fold increase and it showed. The iMac I is now a pleasure to use.

I would suspect your iMac feels slow due to your Fusion Drive. Fusion Drive in the later 21.5-inch iMac models had 24-32GB of flash SSD storage, and a very large, but slower, hard drive. The hard drive is the vast majority of the Fusion Drive, and at times could feel slower.

I would suggest you try booting from an external SSD. If you use an external SSD and run macOS from that, it should make your iMac feel snappier. For more info, instructions, and what external SSD to buy, please read: How to Setup and Use an External SSD as y… - Apple Community.

Cheers,

Jack

Welcome to the Apple Support Communities.

If your Mac is running slow, I believe we need to proceed with an EtreCheck report. EtreCheck is a useful App to diagnose and identify issues, both hardware and software. It is trusted by many users. Please follow the steps outlined below:

1. Navigate to: https://etrecheck.com and get the free version of EtreCheckPro.
2. Download it. It is a free and trusted app. It is often recommended here on Apple Support Communities. 
3. Open EtreCheck, and before running the report, be sure to check “Allow full Drive Access” at the bottom of the EtreCheckPro window.
4. Run the report, and put it in a post, as seen here: https://discussions.apple.com/docs/DOC-250000211. We will then examine the report for issues in your Mac.

Jack

➡️ First, it appears that you do not have a Time Machine backup. It is always a good idea to have one. In order to use Time Machine, you need an external hard drive. Apple sells external hard drives. Here are some options:

• G-Technology G-DRIVE mobile USB-C Portable Hard Drive - Apple (available in 1TB, 2TB, 4TB, and 5TB)
• G-Technology 10TB G-DRIVE USB-C External Hard Drive - Apple (available in 10TB)

Regardless of which drive you choose, it needs to be formatted. Follow these steps:

1. Go to Disk Utility (Finder > Go > Utilities)
2. In Disk Utility, select “View” > “Show All Devices”.
3. Select the top and non-indented drive in the external section of Disk Utility that matches the name of your external drive. 
4. Click Erase
5. Name: any name you want
6. Format: Use Mac OS Extended (Journaled) if you are using macOS Catalina or earlier, or APFS if you are using macOS Big Sur.
7. Scheme: GUID Partition Map
8. Click Erase

Finally, setup Time Machine: Back up your Mac with Time Machine - Apple Support.

➡️ System Integrity Protection is disabled. Re-enable this ASAP!

➡️ Apple Security is disabled. Please go to System Preferences > Software Update, and turn on the following:

• Check for Updates
• Download new updates when available
• Install System data files and security updates

Then, go to System Preferences > Security & Privacy > General, and make sure the "Allow Apps downloaded from" is set to either "App Store" or "App Store and Identified Developers" (that may be the only options shown).

➡️ Next, please check the health of your Fusion Drive using DriveDx and upload the two reports.

Jack

Got it. Thanks!

Have you run Apple Diagnostics: Use Apple Diagnostics to test your Mac - Apple Support?

Cheers,

Jack

Your Fusion Drive is in OK health.

When does your Mac feel slow? What tasks are you performing when you notice a slow down?

Cheers,

Jack