Things to look for when trying to improve upwind boat speed
Part 1: Bend the wind. The wind should be intercepted by the sail and bent to exit along the center line of the boat. Basically, the leach of the sail should nearly parallel the center line of the boat. If the leach is "open", the wind will not be bent as much as it could be and you will not be getting full power from it. If the leach is "closed", the wind may be bent to much windward with the aft section of the sail actually slowing the boat down. The problem is that the top might be "open" and the bottom might be "closed".
The top is controlled by the main sheet tension and the bottom is controlled by the out haul and traveler. A too tight main sheet might hook the upper leach to weather. Too loose and you will not be getting all the power out of the top of your sail. Your indicator is the ribbon flying off the top batten. It should flow most of the time, maybe 50%. 100% flow means that your sail needs to be sheeted in more. 0% means that you have sheeted in too much. Every little puff will change the flow. Constantly be adjusting your main sheet.
Too little out haul will hook the lower leach to weather. This will create more power but also will increase weather helm. Sometimes when the sail looks good you may need to let the traveler out slightly so the sail won't cup so much to windward. This assumes that you have pulled the out haul to remove all the vertical wrinkles along the boom.
Part 2: Balance the boat. Your rudder is a brake. It should follow the boat. Your indicator is the angle the tiller makes with the center line of the boat. It should never be more than 2 inches off center. Cocking the boards up or down will adjust the pressure on the rudder. You should be able to steer with thumb and first finger. The other thing that affects the helm is the angle of heel. The MC scow must be kept nearly flat. Some say 15-20 degrees angle of heel. Definitely keep the rail out of the water and the lee board nearly vertical. Lets assume you have a 45 degree heel. Your rudder is now lifting the stern out of the water instead of turning the boat. Big, big, drag. Keep the boat nearly flat.
Finally, when a puff hits, you may have to ease the sheet a little. But don't let the boat heel.