Boot iMac from external SSD, what happens to internal Hard Drive?

if you want to use an external as a startup disk, then I would proceed as follows:

plug in your new SSD

Launch Disk Utility and format the drive as Mac OS Extended and GUID partition scheme

Download either CarbonCopyCloner or SuperDuper - both are cloning apps

Clone your internal drive (your entire system) to the newly formatted SSD

Choose your SSD as the booting disk in System Preferences > Startup Disk

You now have an exact copy of your current system on the external and can simply work as usual

If you wish, you can then use the internal as a backup drive and clone your external system to the internal to keep both up to date.

From your report, you not only have a snail speed 5400 rpm internal drive, but also only 8 GB RAM which may slow things down depending on which apps you use; I'd suggest you keep a watch on RAM usage and do not run resource hogs like Chrome. Also be conservative on background processes such as continuous updating, uploading, or sharing (which is a security risk) - only run those when you are not doing much else. And, beware of Zoom - personally, I would never allow that on any of my Macs until they have greatly improved their security and rid of the hacking which was made possible by their lack of safety features.

In your initial description you kind of imply that the computer was slow out of the box. Yet, EtreCheck clearly shows problems with things which have been installed. I agree with the other comments here that you should first fix any problems and then you may install an external drive. As to what to transfer to the drive, there is no need to remove all your files from your hard drive unless you choose to.

You might consider what I do. I have a fairly new iMac which has a standard hard drive. (my mistake.) It's not really that slow, but when compared to my wife's new 27 inch iMac with a solid state drive, there is a major difference. I bought and attached a new SSD. I then used Migration Assistant to copy over everything. I now use the SSD's operating system for better performance. I manually keep both drives up to date with all updates which come out for both the OS and all my apps. I also copy any updated file to the other hard drive when changes have been made. I literally keep my back up drive (that is now my Hard Drive) updated to the minute. Everything is duplicated. And I control everything. If a mistake is made, I made it. I prefer it this way because I still don't fully trust any automatic back up systems.

Lastly, I do regular maintenance on my computer: periodically empty browser history, clean out my cache and regularly restart the computer. If you do all that is suggested by the others here, you will speed up your computer. Good luck.

Ah...

Uninstall CleanMyMac per their directions then run etrecheck again to see what chaff it left.

On CMM…

https://discussions.apple.com/thread/251370955?answerId=252661013022#252661013022

https://discussions.apple.com/thread/251442443?answerId=252814053022#252814053022

And see if after removing all the CMM styff if it still reports...

Runaway process - A process is using a large percentage of your CPU.

You still have a remnant of CMM installed:

[Loaded] com.macpaw.CleanMyMac4.Agent.plist (? ff28e0f0 - installed 2020-06-07)

FWIW, when I was using an external SSD boot with my 2013 iMac, I used

the internal drive as a clone of the external. That way if something went a foul

with some update or things just went wonky, I could reboot to the internal

diagnose the problem or simply clone the internal back to the external.

If it's less than 14 days old return it, yes the 5400rpm drives are slow, but not that slow.

App load times will receive a small improvement since the HDD is no longer also handling MacOS temporary files, but they will open much faster if installed on the new SSD.

I am not as experienced as some others who participate here, but the report overall seems to look good. Clearly, CMM has been problematic and you don't need it, The report also noted that you have a number of 32 bit programs installed. With Catalina, they can no longer be opened, so you should either upgrade them or remove them. Other items can be cleaned, but unless you know how to do so, don't worry about them.

Looks very good. main issue handled! :)

You're doing quite nicely, keep up the good work! 👍

From my experience, the load times will be the same, but I would do as woodmeister50 suggested.