iMac running slow. Is it due to large Photos library?

Download and run Etrecheck.  Copy and paste the results into your reply. Etrecheck is a diagnostic tool that was developed by one of the most respected users here in the ASC and recommended by Apple Support  to provide a snapshot of the system and help identify the more obvious culprits that can adversely affect a Mac's performance.

Use the Add Text button to include the report in your reply:

Also assign Full Disk Access to Etrecheck so that it can get more information for the report:

Uninstall Memory Clean. It's counterproductive to the system's own memory management. If you're worried about memory check the Memory Pressure graph in Activity Monitor as you use your iMac. As long as it stays in the green you have no problem.

You've installed software, CleanMyMac 3, which is considered to be tantamount to malware by most of us hear. Uninstall it according to the developer's instructions: Uninstall - CleanMyMac 3.

Also you have IBM Trusteer Rapport installed. It's often required by banks or other financial institutions but is unnecessary from a protection standpoint for Macs. If you can uninstall it and still access your banking institution or it's only recommended uninstall it. It only adds a drain on system resources.

Also remove the files listed in the Clean up section of the report.

Sluggish performance when the CPU is not under stress can be a sign that you need to reset the System Management Controller. Check out this link and see what you think. It gives some simple procedures to try before resetting.

https://support.apple.com/en-us/HT201295

-- Rich

"Is the iMac upgradable? for instance a local firm will replace the disk drive for 500Gb SSD, but I think that might end up being a waste of about £350 "

Definitely, IMHO! Save your money! Your iMac is already 7 years old (nothing wrong with that!) and has only 8 GB of RAM, which is near to the minimum required to satisfactorily run a large, complex system like Mojave. If you can afford it, save your 350 quid and think about getting a new (or re-furbished?) iMac with at least 16 GB RAM and an internal Apple 500 GB SSD, pre-ordered and installed by Apple. I´d definitely not spend any money on your present iMac, but of course that's my personal opinion without knowing your financial situation. Regarding your 150 GB Photos Library...opening that requires some CPU muscle, so you could consider splitting this Library into smaller, more manageable units! Hope this helps you! Regards, Ian.

That's good news. Glad to have helped.

Sorry not to have replied to your last post to me! But now, having just read that your Mac is " usable in a way it wasn't before..." my advice now would be NOT to tinker with your Photos library and try splitting it up as I previously suggested. Leave well enough alone until you eventually get a new iMac! I'm glad OT could help you, and please don't install any AV, cleaning, cache cleaning or "go faster" software...macOS looks after itself. Good luck for the future and a new iMac...you won't regret it! Regards, Ian.