I looked up my times from two years ago when I did the bike and run (walk in my case)at the same tri that I just did.
Two years ago I did not swim, and my bike speed was 12.something mph.
This year I did swim and my bike speed was 14.something mph. Perhaps the new bike? And adrenaline. Definitely part of it.
The walk times were essentially the same.
My goals for next year: cut at least 5 min off swim time. Bike speed 16mph or better. And run the run course instead of walking.