Swimming - feels easier but not faster

I really like the idea of effortless, smooth swimming. Having done a few total immersion classes, that's what I'm aiming for. However, I'd also like to get faster - and that's what it's all about for triathlons - I've yet to see one advertised where they give marks for style.
So I'm hoping to notice some speed improvements over the coming weeks. Today I did 16 lengths (400 metres) in 9 minutes and was overtaken a few times by the old man swimming next to me.
Perseverance is a wonderful thing. I'm all for it, but it's nice to have a few clues that you're going in the right direction.