You asked for some advise so here s what i see

It didnt look like you had any real powered up runs there - legs bent in a saftety stance and you had weight forward rather than driving thru the back foot and fin
you can see the wake of the board starts at about the mast track which means you were not lifting the board up nearly enuf. pull the mast track back a little will help.
to get faster need to straighten your front leg up so that it "pulls up" on the leading edge getting the board to fly more . video looks like you were just cruising as the sail wasnt raked back and you didnt close the gap between sail and board.
as for sail size - you were on a 6.3 which was probably fine earlier but after about 1:30 the wind picked up and you would have been better on a smaller sail.
For a light sailer like you control is way more important than brute force for going fast. if you cant sheet in and lock the sail down its going to slow you down.
I was on a 5.9 that afternoon which was probably the smallest sail out there and did the fastest speeds :)
Pic below shows Carl overtaking Jimbob at LG this year - he has a great stance and check how the wake is coming off his board behind his front foot.
Carl is doing about 5 knots faster than Jim in the back ground