Nothing gets people stirred up here faster than not agreeing with them.
Another case of "My opinion is the only right one" syndrome.
So let's look back at the original post.
The question is about performance and not which is more secure. But instead of answering that people get on their hobbyhorse and start talking about which version is more secure. That was not the question.
If I asked you which car was faster, a Camaro or a Mustang, I'm not interested in hearing that I shouldn't drive the Camaro because the Mustang is much safer. I asked about speed, not safety. If you don't know the answer then say so. If you asked me the way to the bus station and I answered "Purple" you wouldn't be happy.
It is obvious that no one knows the answer to the question and only one person even addressed it. And those that are focused only on security are upset because everyone isn't the same.
And which world are you living in?
Is Linux sued because an exploit has compromised computers running it? Just because it may have been fixed at some point doesn't mean they aren't responsible for older versions, at least by your logic.
Look at all the routers being produced using some hacked up version of OpenWrt 15 or 17. There are thousands of these out there and have you ever heard of any of them being sued because of an exploit? Of course not. Or OpenWrt being held responsible for not retroactively fixing the exploit?
If you wish to focus strictly on router security then more power to you. It is something that needs to be done. But not everyone is going to be that way and getting upset because we aren't isn't going to help.
And getting back to the actual question here, you are so correct. There are many factors to take into consideration when looking at performance. And there seems to no test suite that can be used to measure it.
The only way in most cases is to try each version doing what you intend the router to do. If you want to pass huge amounts of data then test doing that. If Wifi performance is key then test doing that. If you use a VPN check how it handles that. In the world of routers and modems the most looked at test is download speed. Which firmware gives the best speeds on the same router.
I do too. This is something that just doesn't seem to have been looked at for whatever reason. A topic for discussion.