xenforo has only been around for a few years yet they seem to have dominated so many other forum softwares. did you take shortcuts? burning board 4 is desperately trying to copy XenForo and they are still heavily lacking the features and polish and they been around for a long time.
That's what happens when you have people with experience, these guys have basically done Forum Software a good amount of years. You get some practice and insight from that.
You know every single ticket that was filed for vBulletin? Those little patches for this setup, that setup, this issue, that issue? The developers were aware, they prevented a bug from the very beginning
All the optimizations that were done for vB (post views in a different tables, take a look at the query count, smart joins). They know about it. They suffered it.
Smart architecture? Datawriters? Sixth sense about what is a performance hog? They go that too. They invented it.
Couple that with the fact that they are well read people that spend some time actually researching issues and looking for the "right" solution. XenForo has an Object Oriented architecture will a killer plugin functionality, they are pre-parsing the phrases into the templates so that doesn't need to be at runtime, they got rid of eval ...
We have a really interesting thing going in the software development world? The secret is ...
One good person is worth 10 normal people
That is not a joke, I have seen it repeated over and over. The top performers are not twice as good, or thrice as good. They are freaking 10-20 times as good. Hire 10 engineers then? Good luck with that. You cannot substitute talent with extra time and hands.
Then you have some companies that want to hire the cheapest available engineers to get something out of the door. And to ensure quality they hire a QA team, and to ensure that the customers are happy they hire a PR team, and to make sure that everything is well managed they add in two layers of Project Manager and Product Manager. Good luck with that.