Many of the older iMacs still use platter type hard drives and although they may update to the latest macOS many of them are now getting painfully slow. If it was a pc or laptop you would simply take a cover off and replace the hard drive with an SSD version with an increase of at least x 10 thus breathing new life into your machine.
The difference with iMacs is Apple in their wisdom glue the screens on preventing normal users from changing these drives over, as with most jobs we try we have perfected screen removal and replacement, once the screen is removed we clean the machine free from dust and either clone or if not possible replace the old drive with one of your choice and install the latest macOS.
Once the drive has software installed we check the machine works ok and glue the screen back on, all done in house and completed normally within 1 to 2 working days. So if you want your iMac hard drive upgrading drop team Whitedog a line!!