“In the week before we upgraded our primary database server, its CPU usage (from /proc/stat) averaged over 90%: The new AMD EPYC CPUs sit at about 25%.”
Let’s Encrypt serves certificates for more than 235 million websites off a single primary MariaDB server and some read replicas. No sharding, no exotic distributed store. One big box with 64 cores, 2TB of RAM and 24 NVMe drives. Vertical scaling is unfashionable and it still works.