This is simply false. For starters, GitHub actions by default run on Intel Haswell chips from 2014 (in some cases). Secondly, hardware being faster doesn't obviate the need for caching, especially for docker builds where your layer pulls are purely network bound.

"Computers are very fast now" is largely because of caching. The CPU has a cache, the disk drive has a cache, the OS has a cache, the HTTP client has a cache, the CDN serving the content has a cache, etc. There may be better ways to cache than at the level of Docker image layers, but no caching is the same as a cache miss on every request, which can be dozens, hundreds, or even thousands of times slower than a cache hit.