For serious though. TCP has a handshake to make a connection. UDP doesn’t and if you’re going for low-latency, you probably don’t want retransmission anyway, since if you miss the packet it’s too late to do anything about it. If you need the guarantees from TCP, you probably aren’t actually solving a low-latency problem.

Low latency SQL transactions? Not microseconds, but a ms or two is doable unless you have transient connections or Nagle + delayed acks.