RE: to keep speed up
Since you have a fairly wide board, and can plane into and out of your jibes, about the only thing left is for you to concentrate on the pitch trim angle of your board.
Windsurfing boards in general, and the wider ones with more modern flat at the back rockerlines in particular, are quite sensitive to the fore and aft (pitch) angle of attack (AOA).
Get the pitch AOA just right and they leap onto a plane almost effortlessly. Keep the pitch angle just right as you are planing, and the board will gather and sustain very good speeds because with the pitch angle set correctly these boards minimize the drag of the hull over and through the water.
So, when you go into your jibes, it's very critical that you maintian the same pitch AOA into and through your jibes.
You can test this theory on the water very quickly and easily.
Where you place your foot on the downwind side of your board to initiate your jibe has the greatest effect on the pitch AOA, so to test this, simply place your foot a little further forward and at the same place side to side (athwartships or roll angle here).
As you transfer your weight onto that foot to change the roll angle and initiate the jibe, if you've moved your foot too far forward, you can watch as the nose of our board "drops" and the board falls off plane because you have not maintained enough pitch angle for it to continue planing freely. The board sort of "trips" over the rocker transition (where the more curved upward sections forward become flat at the front of the rear planing surfaces).
If you move your foot too far back, you get the opposite pitch "mis trim", but the same overall result as the nose will "pop up" and you will have too much pitch angle to sustain planing and the board will slow down quickly.
So, the secret to keeping your board planing fast through your jibes is to determine where (fore and aft) along the rail that you need to place your rear foot (after removing it from the rear upwind footstrap) to roll your board into the jibe without affecting the fore and aft (pitch) AOA.
Give this a try and I think you will quickly see that getting that foot placed correctly keeps your board planing.
I've suggested to some sailors that they put a small sticker or pad on the board so once they determine where the "sweet spot" is (to avoid the nose dropping or popping up) you have a visual and tactile (if you don't wear booties) indicator as to where the pressure needs to be applied. You could also put a small tape "arrow" along the rail.
Also, try to "roll" your board into the initiation phase of the jibe rather than "stomping or stuffing" it into the jibe.
A smooth transition from going straight ahead to "banking" into the jibe helps alot to keep your speed up.
Hope this helps.