kernel tickrate is a pretty big one, most people don't bother and use what their OS ships with.

Disabling c-states, pinning network interfaces to dedicated cores (and isolating your application from those cores) and `SCHED_FIFO` (chrt -f 99 <prog>) helps a lot.

Transparent hugepages increase latency without you being aware of when it happens, I usually disable that.

Idk, there's a bunch but they all depend on your use-case. For example I always disable hyperthreading because I care more about latency than processing power- and I don't want to steal cache from my workload randomly.. but some people have more I/O bound workloads and hyperthreading is just and strict improvement in those situations.