Mid 2011 27” iMac no longer supported

David is correct, however you should uninstall the ESET "security" and the VPN app as both have a tendency to make Mac OS extremely unstable. They do not add any additional level of security your Mac does not already have. I know they offer a sense of additional security as your computer is no longer receiving Apple security updates, however that sense is false. The only ones that benefits are the companies that you spent money with.

I also agree you should have plan in place to replace the 2011. I replaced my 2011 about 18 months ago and yes mine was also running perfectly. I replaced it because it had a spinning HD which I knew could die at any moment as it was well beyond its service life. In addition I like to stay current on Mac OS and High Sierra is now 4 (almost 5) generations out of date. This means it is only a matter of time before you begin to have problems with software compatibility and if you still have the original HD that may fail any moment too.

I know, not exactly what you wanted to hear however these are points you may want to consider.

The problem is that the computer still works great

Yes, Macs tend to last a very long time. Alas, the software moves on beyond the capability of the hardware. MacOS 10.13 High Sierra is as far as your iMac can go and it is increasingly out of date, indeed 'obsolete' in Apple's terminology. The computer is not supported, which means that Apple will not service it and the OS will not be updated. Because of this the security is also out of date. You may find that Safari, which is updated only with the OS, cannot interact with the website security challenges of 2022 and, as you mention, other browsers could perform better, but still on the foundations of the operating system and the limitations of being released five years ago.

Ultimately you are going to have to replace it for one reason or another, and this might as well be sooner rather than later. Even if you want to keep it a little longer it is now time to plan your transition to a new Mac. A hardware upgrade path is something everyone should have in place.