Sun spider is too old.
It do not reflect complexities of modern JS heavy sites (and there are a lot of them!).
And it fails to distinguish fast/slow. Its simply tooooo short.
That’s one reason why other (Kraken, V8) are better, but also others are based on real code (purified to get JS perf, but still real code).
Do not get me wrong, but claims that browsers do not need fast JS are silly…
Even this comment system use JS heavily.