Soziee wrote:
The iMac has been running so frustratingly slow, hence the ‘upgrade’ but I’m not keen if it’s going to run even worse!
This could be due to a file system issue, a software issue or a failing hard drive.
Use Disk Utility First Aid to check the file system making sure to click on "Details" and manually examine the details to see if any unfixed errors were found even if First Aid reports everything is "Ok".
To check the hard drive run DriveDx and post the report here using the "Additional Text" icon which looks like a piece of paper. If you have a Fusion Drive setup, then post the report for both the SSD & hard drive.
To check for software issues run EtreCheck and post the report here using the "Additional Text" icon which looks like a piece of paper.
If you have a failing drive, then you may be able to install macOS (or clone the existing drive) to an external USB3 or Thunderbolt 2 drive and boot from the external drive. With some types of hard drive failures booting to an external SSD won't be any better as the failing hard drive ties up the system resources due to I/O errors which macOS cannot handle.
FYI, the MBAir 2018+ has a very poor cooling system where the fan doesn't actually move air over the heatsink because they are completely separated from each other (the heatsink doesn't have a heatpipe). Plus the fan has a max speed of 8,000 RPMs compared to the max speed on other Apple laptops of about 6,500 RPMs so the fan noise will be more noticeable.