Fair Enough, Lokito... As you have found out, you cannot easily downgrade MacOS. It can be done, sure, but you basically have to backup the stuff on your Mac, then wipe/erase the hard drive, reinstall an earlier version of Mac OS, then using either Migration Assistant or Time Machine, get the stuff back onto your Mac, if that makes any sense to you.... so, you'd need an external hard drive of some kind, with at least the same capacity/storage as your 2011 iMac, and you'd need to connect it up to your iMac, turn on Time Machine, get it setup and working and do a backup of your iMac. Once it has done that, then you can wipe/erase your hard drive using Disk Utility.... starting from scratch... then, from there, you could use either the restore DVD's (put them into the DVD drive, label side up) or if you had it handy, a bootable USB stick (with the MacOS on it) . for that you'd need a copy of the MacOS installer (in .dmg format) plus a blank USB stick with at least 8 Gigs capacity or greater and LionDiskMaker. that's what worked for me, anyways.... I noticed that you have 8 Gigs of RAM, which is the bare minimum to run MacOS. IF you wanted to spend a bit of money, you could increase the RAM to 32 Gigs Max, which should help some.... Also, replacing the internal hard drive with an SSD would help as well... however, I don't know if you want to spend any money on it or not. I just checked OWC's website (macsales.com) and 32 gigs of ram would cost you $100. It's pretty easy to upgrade the ram, almost any person can do it, and it might require just a phillips screwdriver. As for replacing the built in hard drive with an SSD, that's possible, and it would make quite a difference in your iMac's performance, but that costs money, and as for you doing it yourself, we can't discuss that here...you'd have to seek out that information yourself
good luck
John b