Hi kayccc, I think I have solved this issue. I forgot to post it here, so sorry!
ExecutionContext::Enqueue is an asynchronous interface, it will return immediately after pushing tasks into the internal queue. So the inference_time is not the duration of this interface.
Also thanks to AastaLLL for the informative comment.
But actually, I have another unsolved issue on how to count data I concerned from sqlite DB, please see here. Would be greatly appreciated if anyone replies to it.