"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.