Assuming this is your iMac:
https://everymac.com/systems/apple/imac/specs/imac-core-i3-3.2-21-inch-aluminum-mid-2010-specs.html
it did come with a 1 TB, 7200 rpm hard drive, which was/is a "decent" speed in terms of rpm. But if you have neglected to do any disk cleanup/maintenance/repairs (from a software perspective), and which could contribute to the drive going bad, that could explain why things are bogging down.
Certainly, an SSD would boost the performance, and the worse shape that HDD is in, the more dramatic the performance boost would be. But it might help if you can give us insight as to what you use the machine more.
It might be good to try and do as much disk cleanup/maintenance/repairs as you can, which could help in determining whether the drive is going bad or not. Here are some excellent freeware products you can use to help with that:
Onyx - Available from here:
https://www.titanium-software.fr/en/onyx.html
Excellent, solid product. I depend upon it (have been using it for along, long time), along with the excellent commercial program Tech Tool Pro. Just make sure you get the correct version of Onyx for Yosemite.
GrandPersepctive - Available form here:
http://grandperspectiv.sourceforge.net/
Another gem. Nice, slick way to see graphically what files are taking up space.
AppCleaner - Available from here:
https://freemacsoft.net/appcleaner/
Fantastic product for removing most files associated with an application that you are removing.
Malwarebytes - Available from here:
https://www.malwarebytes.com/mac/
Very good software for seeing if you have any malware/adware, etc. on your machine.
It's important to realize, though, that you might need a more robust disk cleanup/maintenance/repair program, like Tech Tool Pro or Disk Warrior. Apple's Disk Utility program is good also (comes as part of the Mac OS), but is not as "extensive" as the two I mentioned. To use Disk Utility effectively, your iMac would need to be booted in an "isolated" manner. For that, you could boot your iMac via the (hidden) Recovery HD partition. This link describes how to do that:
https://support.apple.com/en-us/HT201314
However, as I mention below, it is imperative that you have a backup first, before doing that.
Another culprit regarding disk space is deleted EMails. It's important to remember that when one deletes an EMail from their machine, it is not permanently removed. I use Outlook 2016 for my EMail client, and there is an easy way to have deleted EMails permanently removed (I use it just about every time).
Finally, BEFORE you attempt any kind of maintenance, it is
critical that you have been making backups to an external device. Hopefully you have. If so, what software do you use for that?
