The dbcache *is* a read cache, and this misconception has kept block validation times longer than they need to be. This problem will be fixed in v28. https://github.com/bitcoin/bitcoin/pull/28233
Today is a good day to publish some super nerdy research!
https://blog.lopp.net/effects-dbcache-size-bitcoin-node-sync-speed/
Discussion
Setting higher dbcache benefits block validation time in addition to initial sync time. It should always be increased if there's enough free RAM.