Most people converting to TI report that it really takes 6 months to a year before it really "kicks in." This seems especially true of more experienced swimmers, where newer swimmers have less to "unlearn."
I would hang in there. It does seem like significant speed increases esp. at the skill level you are talking about does take some hard work and concentrated effort, whereas the "do a few drills for a little while and I can survive a triathlon" people get satisfactory results relatively easily.
Swimming really well is hard work physically AND mentally.