Am I correct in thinking as far as once off question / response they are equivalent but the thread API allows a dialog, several an interactive back + forth on the same problem/context
Assistants reduces the code burden on the developers as it handles RAG and conversation threads internally, however, you can do all of that yourself with greater granularity with the vanilla API.
To engage the speed question a bit as that has been on my mind with 2-10 seconds roundtrip (including tool/function call in middle) using assistant/thread API.
Do you mean the assistants/thread API or Vanilla API could be at greater speed, I think former but am not clear.