HELP! Macbook Air 2017, Was fine one day and when turned on the next it runs slowly!

I think there are a couple of things going on.

After doing the factory reset, the computer has to do a lot of initial setup again. That is the heavy CPU usage you are seeing. Some of the processes are system processes running as the root user. The Mac App Store cannot see these processes directly so they show up as “Other”. But photo analysis is showing up, so that is what leads me to make this conclusion.

I’m afraid you will just have to wait until these processes finish up.

What is actually causing the slowdown is your hard drive. The read and write speeds are at least half of what they should be. Your file system test failed to complete and was terminated. This usage could be caused by those setup tasks.

Once the computer settles down to a point where the CPU stay at 90% idle or more, run another EtreCheck report. If you see massive improvements in your hard drive performance, then the problem should be resolved. But if you continue to see poor performance, then perhaps it was a failing hard drive the whole time.

A Troubleshooting Procedure that may Fix Problems with macOS El Capitan or Later

You should try each, one at a time, then test to see if the problem is fixed before going on to the next.

    If possible, backup your files before proceeding.

1. Shutdown the computer and disconnect all third-party peripherals.
2. Wait 30 seconds before you restart the computer.
3. Resetting your Mac's PRAM and NVRAM
4. Reset the System Management Controller (SMC)
5. Reset your Startup Disk and Sound preferences, if needed, after resetting the PRAM.
6. Test in safe mode to see if the problem persists, then restart normally. See How  to  use  safe  mode  on  your  Mac.
7. Repair the disk by booting from the Recovery HD. Immediately after the chime hold down the Command and R keys until the Utility Menu appears. Choose Disk Utility and click on the Continue button. Select the indented (usually, Macintosh HD) volume entry from the side list.  Click on the First Aid button in the toolbar. Wait for the Done button to appear. Quit Disk Utility and return to the Utility Menu. Restart the computer from the Apple Menu.
8. Create a New User Account Open Users & Groups preferences. Click on the lock icon and enter your Admin password when prompted. On the left under Current User click on the Add [+] button under Login Options. Setup a new Admin user account. Upon completion log out of your current account then log into the new account. If your problems cease, then consider switching to the new account and transferring your files to it - Transferring files from one User Account to another.
9. Reinstall a Combo Updater  macOS Catalina 10.15.6  Combo  Update, macOS Mojave 10.14.6 Combo Update, macOS High Sierra 10.13.6 Combo Update, macOOS 10.12.6 Combo Update, OS X El Capitan 10.11.6 Combo Update.
10. Reinstall OS X by booting from the Recovery HD using the Command and R keys. When the Utility Menu appears select Install OS X then click on the Continue button.
11. Erase and Install OS X Restart the computer. Immediately after the chime hold down the Command and R keys until the Apple logo appears. When the Utility Menu appears:

     1. Select Disk Utility from the Utility Menu and click on the Continue button.

     2. When Disk Utility loads select the drive (out-dented entry) from the Device list.

     3. Click on the Erase button in Disk Utility's toolbar. A panel will drop down.

     4. Set the Format type to APFS (for SSDs only) or Mac OS Extended (Journaled.)

     5. Click on the Apply button, then wait for the Done button to activate and click on it.

     6. Quit Disk Utility and return to the Utility Menu.

     7. Select Install OS X and click on the Continue button.

The SSD in a MacBook Air is not replaceable. You can always take the computer to Apple and have them do diagnostics. If they determine that the SSD is faulty, then they will replace it. But if you don’t have an AppleCare extended warranty, then you will have to pay for it. And the problem could be the motherboard rather than the SSD, which could cost much more.

Since you are still seeing the problem, you can run Activity Monitor to see what those other processes are. Make sure to set it to view All Processes in the View menu. Another option would be to use EtreCheckPro instead. Since EtreCheckPro is not in the Mac App Store, it can directly see these processes.

EtreCheck doesn't fix anything. It's a diagnostic tool.

I've already posted my suggestions. Either none of them worked or you decided not to try all of them.

Katkins99 wrote:

I've seen a few videos of them being replaced, ...

You can watch those videos here https://eshop.macsales.com/installvideos/macbook_air_2015/?filter=ssd

... but it's far from certain a replacement SSD will fix whatever is wrong with your Mac. SSD failure may very well be the cause of your MBA's poor performance but failures isolated exclusively to it are rare. I'd get a conclusive diagnosis from Apple first.

Until then follow etresoft's suggestion:

Since you are still seeing the problem, you can run Activity Monitor to see what those other processes are. Make sure to set it to view All Processes in the View menu.

  Other processes 143.86 % (?)

Identify those "other processes".

Katkins99 wrote:

I've seen a few videos of them being replaced, is it a case of its not recommended to do it yourself?

Technically speaking, any repair is possible. But if you do it yourself you will void the warranty and Apple may even refuse to service it for any price after that. Plus, the problem may not be the hard drive at all. As John Galt says, the problem might not actually be the hard drive.

Also will etrecheckpro actually fix my problem or is it a waste of money?

EtreCheckPro is free. It will just make it easier to see what other "Other processes" are.