If this is your Mac:
https://everymac.com/systems/apple/imac/specs/imac-core-i7-2.93-27-inch-aluminum-mid-2010-specs.html
it came with a 7200 rpm drive, which is not bad. However, High Sierra (and most likely Mojave) runs better on Macs with SSDs. Also, if little to no disk cleanup/maintenance/repairs, from a software perspective, has ever been done on the drive, that will also contribute to the slowness.
How much free space is on the drive?
As for going back to Sierra or El Capitain, you'll need first to download the applicable installation file from the App Store. Once you do that, make a copy of it in another location. It will first be inside your Applications folder.
You can actually do a good amount of disk cleanup yourself. One place to look is your downloads folder. Also, here are some free programs you can use to clean up/maintain/repair the drive:
Onyx - Available from here:
https://www.titanium-software.fr/en/onyx.html
Rock Solid, has been around a long, long time, and has had many positive reviews. I use it religiously (along with a commercial program entitled Tech Tool Pro). I would not be without either of them. In fact, I will not upgrade to a new Mac OS until there are compatible versions of both of them for the new OS (along with 4 other critical programs that I have). Note that there are different versions of Onyx available, for each different Mac OS. You would need to get the correct version for the Mac OS you are using.
AppCleaner - Available from here:
https://freemacsoft.net/appcleaner/
Another stellar performer! As you can see via that link, it will get rid of most stuff associated with an application that you wish to delete. Like Onyx, note that there are different versions for different versions of the Mac OS. You would need to get the correct version for the Mac OS you are using.
GrandPerspective - Available from here:
http://grandperspectiv.sourceforge.net/
Nifty graphical way to see which files are on your drive. Good way to identify large ones that you might not need.
Malwarebytes - Available from here:
https://www.malwarebytes.com/mac/
Good to use if you see adware and/or suspect you might have a virus. It also is good to run it every so often, even if nothing is wrong.
ClamXAv - Available from here:
https://www.clamxav.com/
More extensive than Malwarebytes in terms of finding viruses. You can use it in demo mode, and it still has just about full functionality.
Then, you should consider using either SuperDuper! or Carbon Copy Cloner to make a bootable backup:
SuperDuper! -
https://www.shirt-pocket.com/SuperDuper/SuperDuperDescription.html
Carbon Copy Cloner -
https://bombich.com/
You can use either them in "demo/trial" mode, although I believe you can use SuperDuper! in trial mode for more than 30 days. Using either of them will make an installation of El Capitain or Sierra much easier. And you will have way more control over the process.
Also, once you make such a backup, and then boot from it, you can use Disk Utility there to analyze your drive, and if necessary, repair it.
