Single-core performance is largely topping out. The gains are incremental at best.
Mobile SoCs have gone incredibly multi-core incredibly quickly (6-8 cores are common!). Meanwhile the web is incredibly bad at handling that. JavaScript code still has to fight with the GC for CPU time on the single thread they both occupy. WebWorkers are incredibly heavy & limited.
WebAssembly might have threads eventually (it's on their roadmap), but then how long until that happens and will the typical webapp ever even migrate to webasm?
Meanwhile CSS & DOM performance continues to be poor, so webapps are left in this awkward state of needing to re-write part of the browser.
Hardware isn't fixing any of this. It's been almost a decade of hardware not fixing this at this point.