There's a whole skit in the vein of "What have the Romans ever done for us?" about ZeroMQ[1] which has probably lost to the search index now.
As someone who has held a socket wrench before, fought tcp_cork and dsack, Websockets isn't a bad abstraction to be on top of, especially if you are intending to throw TLS in there anyway.
Low level sockets is like assembly, you can use it but it is a whole box of complexity (you might use it completely raw sometimes like a tickle ack in the ctdb[2] implementation).
https://en.wikipedia.org/wiki/Constrained_Application_Protoc...
It is UDP-based but adds handshakes and retransmissions. But I am guessing for your benchmark transmission overhead isn't a major concern.
Websockets are not that bad, only the initial connection is HTTP. As long as you don't create a ton of connections all the time it shouldn't be much slower than a TCP-based socket (purely theoretical assumption on my part, I never tested).