How to improve old, slow iMac -- new SSD? Or more RAM?
According to your comment, it looks as if you could do better with more RAM. If it was just launching slowly, then I would say the SSD, but since its switching apps also, I think more RAM would be nice. Unless your iMac ran a lot better under Mountain Lion or below, I really wouldn't downgrade.

BTW: Don't disable the user accounts. Just log off when you are not using that user account.

Since you have a spare SSD, you could also just connect it externally using an inexpensive USB to SATA adapter and install new OS on it and run some A/B tests. You won't see the full speed of an internal placement - especially if you use USB 2.0, but it's often enough to know rule out RAM or OS as a cause of slowness.