The folders on my iMac had all their icons changed to look like a blue sack with a red belt around it! Is this a security issue

No, sorry, I can't. The folders had returned to normal the next day, yesterday. I've never seen that icon before so I was a bit alarmed by it. The machine runs very slowly sometimes – I had to restart just now to post this – but otherwise it seems unchanged.

Thanks for that reply, BDAqua. I've done as you suggested, and the Disk First Aid did seem to fix the immediate slowness problem, but it inevitably recurs so I also did the EtreCheck diagnostic. I'm just slightly offended that it had to mention that my late-2013 iMac is, in Apple's estimation, obsolete  :(

I did install TunnelBear years ago and thought I had removed it but found it again today. Now I’ve moved it to the bin and emptied it but can’t be sure it’s all gone. They don’t provide an uninstaller.

As for antivirus software, I installed Malwarebytes years ago and it’s the same story: thought I had deleted it but can’t be sure.

I have these questions…

1 There were unsigned files and launch daemons from known applications, none of which sound dubious but I deleted them as I don’t use them: MicroSoft, Adobe and TunnelBear .plist files. Then I ran a second EtreCheck, which is the one attached as extra text.

2 I think I read somewhere that Java is verboten and to delete anything to do with it. I saw a file called com.oracle.java.Helper-Tool.plist – should I delete that?

3 I deleted a font app using their uninstaller, but there are 8 others listed as unsigned apps, all in Library > Scripts > ColorSync. Do they need to be deleted? ColorSync is part of the OS, isn’t it?

4 I turned off File Sharing and Remote Login.

5 As for the “Minor issues” listed – Heavy RAM usage, Apps with heavy CPU usage, Heavy I/O usage, Kernel extensions present – I have no idea what these even mean, so I do need your assistance here, please!

6 I haven’t used Time Machine for maybe 5 years or so, so I doubt it would be involved in this problem.

Thanks again. Now I'm worried! Will try Drive DX although I'm wary of installing extras at this stage; only because you recommend it.

Another thing: although I thought I had turned off Remote Login, I found later that it's still on. It hangs.

I saw a post by alb- in these forums who had the opposite problem: a hang trying to turn RL on. This user could turn RL on after reinstalling a basic Catalina without updates but it hangs with 10.15.7, my OS. Someone suggested starting in safe mode, but that didn't make any difference for me.

I've read that this can be dangerous or invite trouble. Can you suggest a solution for this?

Thanks for your input, g_wolfman, much appreciated.

I've done a few things today and gotten some bizarre crashes and failures. But I had Terminal open to try a few things I'd read, and eventually decided to follow Terminal's advice to change the default shell from bash, I think it was, to zsh. This means nothing to me ;) but it seems it was introduced with Catalina which I'm running. I also deleted a lot of Launch Agents and all the Skype files I could find, but I doubt that would make a huge difference. Anyway, I did a new EtreCheck and the Swap Used has gone from 1.28 GB to zero.

Would the change to Terminal explain this?

I had a lot of problems trying to log in with the new account; thought it would totally crash and never revive. It started working hard with a whirring noise and heat coming from the disc area, which it has done occasionally in the past.

Then I changed the Terminal default shell from bash, I think it was, to zsh and deleted some files I didn't think I needed, and did a new EtreCheck; see my reply to g_wolfman, above. Now I've also run DriveDx which reported 0 issued found, overall good, overall performance good and SSD lifetime left indicator average. I'm so relieved! Report is attached. I've changed the name of my computer to xxx and hope there isn't any other personal info in there.

Please let me know if it looks alright to you, and what you recommend now, if anything. Thanks in advance for your precious time!

Just realised there were actually two DriveDx reports generated, both under the same computer name. Don't know why that is, but this is the second one.