2017 iMac slow and is prone to "Beachballing"

To turn the security back on is pretty straightforward, however I would recommend that you call AppleCare ( AppleCare Contact Info ) and they can help you over the phone. They will also help you to get your Fusion Drive un-split or you can follow the steps in How to fix a split Fusion Drive - Apple Support

If you want to keep the machine, yes you could use an external SSD as your startup drive. Follow the directions in How to setup an external SSD as your startup disk. The external SSD I would recommend is the OWC Envoy Pro SX.

That all being said, another option is rather than investing in an obsolete and vintage 2017 iMac consider replacing it with a new Mac Mini which will be orders of magnitude faster. All you will need is an external display. I would recommend getting the base model with 16GB of RAM and a 1TB SSD. External displays start below $100 however for a good quality one, expect about $300. So for about $1300 you have a new computer and display, a 1 year warranty, the machine is eligible for AppleCare and it will far outperform your old device.

I suspect the Fusion Drive has become infused, possibly a failingHD. Also, somehow the Security has become turned off, which I have seen before when Fusion Drives become infused. \

The Apple security being turned off is a very serious issue too!

These scores for your Fusion drive system:

Performance:

System Load: 1.57 (1 min ago) 1.80 (5 min ago) 2.37 (15 min ago)

Nominal I/O usage: 0.03 MB/s

File system: 26.29 seconds

Write speed: 763 MB/s

Read speed: 1148 MB/s

are just a bit under expected speeds but not slow enough to typically cause issues in normal use. Nominals for your Fusion configuration are Writes between 600 and 900 MB/sec, and Reads at about 1300MB/sec and up. Also your File system score of 26+ seconds is excellent and tells me the drive system is heathy. I worry if the File system scores go over about 45-50 seconds. Anything over about 70 seconds is a Big Red Flag.

Unless you go with an expensive thunderbolt external SSD. your current scores are much faster than the typical low-cost external USB3 SSD workaround, which can do no better that 400MB/sec for both reads and writes. Replacing the internal mech hard drive with an SATA 6GB SSD gets you only to 500-600MB/sec. You are already ahead fo those.

I think you have some interferences installed that are not Apple's.

—One VPN is hard enough on system performance. You have FOUR running 😱 and a fifth waiting in the wings. That avarge user does not need even one. Most of the ones over-hyped in TV adverts and popups are close to useless and some actually degrade security. Shut the down and see if drive scores improve.

—OneDrive can be a little too active, and can slow performance if you have a slowinternet upload speed. If your upload speed is about 50MB/sec or higher, don't worry about it.

—Less of a suspect but worth testing is the external USB3 Seagate drive. Seagates are good, but I have seen Etrecheck reports where speed improved after disconnecting a USB external. Easy enough to test.

—This little disturbance in the Force that shows in the report:

2024-12-16 20:31:36 photoanalysisd High CPU Use

Executable: /System/Library/PrivateFrameworks/PhotoAnalysis.framework/Versions/A/Support/photoanalysisd

is easily managed. Please see:

https://apple.stackexchange.com/questions/310594/what-is-photoanalysisd-and-why-is-it-using-77-of-my-cpu

Your welcome.