Not sure - I don't as a rule trade US stocks, so I'm not au fait with the fill times, although it will of course depend on which stock you're trading. Even with direct access, not everything gets filled immediately. I watched Alan Farley trade using direct access, and he was filled in batches too.
Perhaps one of the clever guys will know more.
I had a root around a year ago to find day trading possibilities of US stocks in an ISA wrapper, and the best I could come up with at that time was myBroker.