How do I find and remove x86-only apps?

You don’t need to worry about that since you have an Intel Mac.

However, I see you ran EtreCheck for “beachballing” issues. If that is the case, 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: https://discussions.apple.com/docs/DOC-250003583.

Also, make sure you have a backup strategy in place!

Jack

I think i’m going to give the external SSD it a try! Would this be suitable?

https://www.amazon.co.uk/gp/product/B08F27QGHX/ref=ox_sc_act_title_1?smid=A3P5ROKL5A1OLE&psc=1

No. My Passports do not typically function well as bootable devices. In the User Tip I linked, check out the G-DRIVE SSD. Here it is on the Apple UK Store: G-Technology G-DRIVE mobile SSD R-Series Storage - Apple (UK).

Jack

That would be up to 560MB/s if older SSD and the USB-3 port if Gen 1 can support theoretically up to 5 GB/s throughputs.

No that WD drive is USB-C and your 2015 iMac has USB 3 ports that only support 560GB/s speeds.

You will need to get and use an external SSD drive that supports USB 3 or at least has cables for both USB 3 and USB-C.

Yea' my bad, a brainfartypo error on my part. 560MB/s not GB/s.