Browsers aren't single threaded, so the ST performance isn't the end-all-be-all metric to use. JavaScript is single threaded, but the browser itself uses multiple threads. E.g. there is the main thread, which handles the user input. There is the rendering thread, which actually takes up a majority of the CPU time on a lot of websites. And then there is stuff like GPU acceleration, which makes a lot of the web performant when it wouldn't be otherwise. Of course, a server does all of these things worse than a decent laptop.

It is the end all, be all metric. The reason is because once a certain threshold of MT performance is reached, which the M1 most certainly reaches, then the ST is what matters for how fast your website loads, even with multiple tabs opened.