Is there a way to clear cache memory on an iMac?

Welcome to Apple Support Communities.

If your iMac is slow, clearing cache manually is not the solution.

To see why your iMac is 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.

Cheers,

Jack

+1 on restarting the computer more often than every three weeks.

These are fairly normal drive scores that under-spec drive, but are too slow with newer macOS versions:

Performance:

System Load: 1.74 (1 min ago) 3.49 (5 min ago) 3.21 (15 min ago)

Nominal I/O speed: 0.05 MB/s

File system: 33.70 seconds

Write speed: 69 MB/s

Read speed: 65 MB/s

Had Apple opted to fit a more expensive SATA 6GB, 7200 rpm drive, scors would be roughly 2.5X faster.

You are hurting yourself by using Chrome on a computer with only 8GB RAM that cannot be upgraded. Chrome's hunger for resources in apparent in this section of the report:

Top Processes Snapshot by Memory:

Process (count) RAM usage (Source - Location)

Google Chrome Helper (Renderer) (23) 2.01 GB (Google, Inc.)

EtreCheckPro 262 MB (Etresoft, Inc.)

Google Chrome 216 MB (Google, Inc.)

kernel_task 166 MB (Apple)

MTLCompilerService (28) 162 MB (Apple)

Its "helper" laid claim to 25 percent of your memory at the time of the test. More frequent restarts can help keep that hungry hippo in line.

First, you need to have a Time Machine backup. If you do not have a Time Machine backup, I recommend using a SanDisk Professional G-DRIVE - Western Digital as your backup drive. You can then follow the Apple Support steps to setup your Time Machine backup: Back up your Mac with Time Machine - Apple Support.

I personally also use iCloud in addition to Time Machine to sync all my files and photos to all my Apple devices. Although iCloud is not a "true" backup service, iCloud can also act as an emergency backup, but this is why iCloud should be used in addition to Time Machine. Here are some resources on setting up iCloud for Mac:

• Upgrade to iCloud+ - Apple Support
• Set up iCloud Drive - Apple Support
• Add your Desktop and Documents files to iCloud Drive - Apple Support

However, the reason your iMac is very slow is due to the 5400 rpm hard drive. 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.

I would also recommend restarting your Mac once a week.

Cheers,

Jack

To answer your question, start in Safe mode

Start up your Mac in safe mode - Apple Support

I would skip TM until the performance issue is resolved. If/when it is, run it once a quarter instead of all the time.

For starters, uninstal Malwarebytes and see if that improves things. If that doesn't work, try a full erase and re-install as new (manual copy of user data to external disk prior). Then re-instal manually your applications in sequence to see the impact on performance.

Do also consider a new iMac vs. the cost of an external SSD and related effort. IMO well worth it, especially since your iMac is fast approaching vintage status.

Good luck!

I agree with Jack-19. I would try a clone on an external SSD. I doubt Malwarebytes has any speed impact on your Mac.