Upgrade possibilities questions - please help!
I have a late 2015 imac 21.5" 8 GB 1867 MHz DDR3 RAM, 3.1 GHz Intel core i5 processor. Computer is about 2.5 years old, but I've never been really happy with performance, especially compared with a 2008 imac which I got a good 8 years out of. Very slow multitasking, moving between apps when multiple apps open, slow to get going when awoken from sleep and even slow sometimes on just changing browser tabs. Some of these seem even slower than the 2008 which I still have around, but don't really use. The slowness is really frustrating, but I don't really want to get a new computer yet since this is less than 3 years old, so I'm wondering if there are some easy fixes or upgrades I can do (or people can recommend) so that I might be able to get more time out of this current imac.
From my reading, its obvious that I cannot upgrade the RAM on this particular imac I have. But I have read some posts about installing and running the OS from an external SSD, which some people say works well. I currently run Mojave, though don't think the slowness got significantly worse when upgraded from Sierra, though it is getting worse in general.... Would running the OS off an SSD drive be a feasible option, and would it make a significant improvement in performance?
If not, would doing a hard factory reset potentially help?
On a kind of separate note, I read a couple of threads mentioning using an old mac as a second monitor - is this something I would be able to use my 2008 mac for?
Any thoughts, advice, suggestions greatly appreciated!
iMac 21.5", macOS 10.14
