While in System Preferences, go to the Spotlight pane, and add your SSD to the Privacy window. Go to System Preferences, and set your boot drive in the Startup Disk pref pane, as NVRAM reset clears that selection. Continue to hold the same 4 keys until you hear the boot chime two more times, then release the keys to let your system boot normally. Reset NVRAM (hold Option-Command-p-r on a restart. (the reinstall does not delete any of your own files and added apps and settings, unless you choose to erase the SSD first) That will simply reinstall all the system files, and can sometimes give you back that performance that may not be there just yet. If you simply did a full restore from a backup, I would suggest that you boot to your High Sierra installer, and reinstall High Sierra to your SSD. What process did you use to transfer your system and files from the hard drive to the SSD? Late-2014, Intel Core i7-4578U (4,750), non currently on eBay UK (dual core) ( £1,130 new from Apple)

Late-2012, Intel Core i7-3720QM (8,120), non currently on eBay UK (quad core)

These are the starting prices for used ones on eBay UK (PassMark CPU scores in brackets):

Blooming heck, I didn't realise how expensive second-hand Mac minis (that supported macOS 10.13) would be!

Mac mini mid 2010 ebay