Caching at all levels is key to good db performance (cpu cache <-> ram <-> disk). CPU cache optimization is not my area of expertise, but I did write another fun article on io devices and how it related to databsae perf:

https://planetscale.com/blog/io-devices-and-latency

Network saturation, and just resource saturation broadly, is a huge reason to shard. If/When the network, cpu, or disk for a subset of shards becomes a bottleneck, add more shards.