Comment by sgarland

Having indices doesn’t guarantee anything is cached, it just means that fetching tuples is often faster. And unless you have a covering index, you’re still going to have to hit the heap (which itself might also be partially or fully cached). Even then, you still might have to hit the heap to determine tuple visibility, if the pages are being frequently updated.

Also, Postgres has supported parallel scans for quite a long time, so single-core performance isn’t necessarily the dominating factor.