My top speed for the day often is in the first run, where I'm not comfortable yet, often things aren't balanced and I'm not trying for anything special. I think, oh this is great, I'll be able to improve a lot on that when I am trying harder, but it doesn't happen. Why is this so?
happens to me sometimes as well, fresher, stronger 
Maybe start to sail defensively or have bad habits
Quite often the more you think about it (in later runs) ther more it gets in the way. I also find that all the things I've done in a previous session tend to consolodate after a break and all the good stuff comes out the next time out.
Sometimes your first run is in the best wind, has happened to me a few times.
But last time I was out, my last run was the fastest, after I'd got everything balanced.
That can happen, used to notice in the Canberra easterlies, if you're last out, everyone else has packed up, the water gets flatter.
Sometimes the first run is the best wind because the best of it has already passed by the time I get out, however the first run is so often the best that there must be more to it.
I have had similar experience in the past, first, or last run with not much in between.
Since being ill and not knowing if I would ever windsurf again, then recovering and getting out I seem to have relaxed a bit on the water and am not going out to push the limits of my sailing. On 3 out of 5 sessions I have had some of my best speeds (only had 33+ twice before) with 33knots, 32knots and 31 in bugger all wind - wind I would have expected 30 at best.
Perhaps its something to do with just relaxing and not pushing too hard, almost letting the board do its stuff while you just tweak it rather than forcing it. Try it, just sail, no thinking or looking at your GPS. Go with a gust rather than forcing the board with the gust.
I think it is the extra weight you carry on the first run, you haven,t had time to cr*p yourself yet.
