Fitting and SSD will make a MASSIVE difference regardless though you might need an app afterwards to stop the fans running at MAX.SSD upgrade is defiantly worth it. I have a 2009 imac that I still use a lot after doing the SSD upgrade and adding even more RAM (12GB)
Mine is a 2015 model and I have noticed that after some of the recent OS updates it now takes significantly longer to boot up. It seems to hang for a significant time during the process. Before adding memory or swapping to a SSD I would wait to see what discussions take place on the forums. It may be something that will be sorted in the next update.
Back up everything with time machine first! And a make a second copy of important files, photos etc. If time machine disk gets full it'll automatically clear out the earlier back ups.
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