Sherlock was lurking in the kitchen, where part of his informant network worked as a sous chef. He had no intention of arriving until the ladies did, and even then it would not be rushed.
He really didn't get enough credit for not going on killing sprees. This was intolerable. Sherlock Holmes did not date. He certainly didn't double date. What was he supposed to do? He had literally no interest in the woman, and frankly if Watson hadn't asked he would have had a rather impolite response when asked if he wished to spend the evening with subpar Chinese food and worse conversation.
But John had asked, and as Sherlock had precious few friends he was willing to bother with, he agreed rather than risk irritating one of them.
It was still stupid, though. Even their General Tso's was mediocre at best.