What refresh rate are you running at? I've noticed that the SD card is pretty slow, so writing to it too often slow down the whole thing.
I'm also thinking that a SOG indicator is in order. The GPS produces that, but it's not continuous enough to be able to play optimization games with sail position.. At that point, a wind speed indicator would become necessary, as you'd need to know whether a speed up/slow down was the result of a sail change or a wind change..