Many trading companies now utilize over-the-horizon radio. Its the fastest, most direct connection available. But even without this:

- cpu thermal profiles and power governors

- cpu affinity

- non-evented io(if you prefer latency over the number of clients served)

- network interrupt coalescing(actually, disabling coalescing), network msi-x, hardware offloads, 10gb fiber or DAC connections(very cheap nowadays)

- lock-free and kinda-crdt data structures(between threads)

- predictable memory allocations(better - no allocations on critical path)

- specialized logging and tracing(no stdio)

can drive you quite far.