I know for example the devtool is written in react and it's a lot slower than chrome devtools (I don't know how chrome devtool is written) and the javascript debugger is sometime not even usable because it's so slow.
I find it mildly amusing that there's a profiler in the devtools. And yet there is another profiler that is at times more powerful than it. I have used the gecko profiler from time to time to identify which feature the page was using that slowed down the entire browser. Multi-process means just debugging a single tab doesn't give you the whole picture.
To any other devs who like me who want to use Firefox Debugger, but were driven back to the devil Chromium by need for consistent, seamless copy-paste and select-middlemouse interaction: Give 70.0.1 a try. Much, much better. So much that I've dropped Chromium. (Ubuntu 18.04)
Using web components or something else is only one small part of the performance question. They could be using both web components and React for all we know. They could be using any number of JavaScript libraries that could be harming performance. They could be using raw DOM manipulation which could be either very fast or somewhat slow depending on how well it was implemented. All we can say is that using JavaScript in general is slower than using C++ but they weren't using C++ before anyways.
chrome devtools are also written in javascript (although I'm not sure which framework if any they use). You can even debug the chrome devtools with chrome devtools (I think the same applies to firefox, although I've never accidentally gotten into that state like I have in chrome).
I know for example the devtool is written in react and it's a lot slower than chrome devtools (I don't know how chrome devtool is written) and the javascript debugger is sometime not even usable because it's so slow.