I had very similar issues out of the gate with my

Coming from a Columbia Sabre '32 that I could easily and routinely get to 8 knots, I had a rude awakening with the Mac. Basically I found out that I didn't know that much about sailing.
Here are the points that I've really had to learn:
1) Do not pull the jib through a tack until you are on your new course. Leave it back-winded all the way through the tack. Pulling it through before that will de-power the boat and drop speed to near zero, and you may find that you can't make the tack. Simply coming around completely through the tack before resetting the jib will solve that problem.
2) Over steering the helm stops the boat. On these boats, the rudders and the motor are linked and they turn the same, but they have different purposes, so the helm can go way farther over than the rudders should ever be turned. When they go beyond about 30 degrees, they become sideways boards that act as brakes and really reduce forward momentum. Worse, because the helm initially seems really sloppy, I tended to over steer from side to side, compounding the problem. When under sail, I keep my turning circles wide--essentially no better than what I could do with a tiller. Under power, you can turn these boats on a dime and that's nice, but there's no stops to prevent you from braking the boat by oversteering with the helm.
3) The boat REALLY wants to be at a 20 degree heel. It doesn't not have the rounded hull of a true sailboat, so it has varying hull efficiencies at different heels. At 45 degrees, you're loosing power. The trick to this one is to sail with the mainsheet in your hand, either uncleated or with a procedure to really snap it out of the cam-cleat quickly, and just use constant tension to allow gusts to spill and tighten in immediately when the sheet feels slack. Spilling and tightening in real time has done the most to get me above 5 knots consistently.
4) The boat's theoretical hull speed is 6.4 knots. A great sailor won't do better than that in these boats routinely, although there's plenty of footage of people planing downwind up to truly ridiculous speeds.
5) If you're using a roller furling Genoa, understand that it's not the most efficient sail in anything but light airs. I usually run with mine furled to 100% or less vice it's full 150%. My rule is basically this: I start on the main only, get sheeted with the main and vang to maximum speed, and then check that speed. If I'm doing 3 knots or below, I pull out the full 150%. That usually gets me to the 20% heel sweet spot, and then I can spill wind with the mainsheet to keep the heel right. If I'm doing 4 knots, I'll pull it out to 100%, and if I'm doing 5 knots, I only bring out 70% or less. If I'm already at 6 knots, I only bring out a patch to compensate for helm balance. The point here is to set the Genoa so that you average that 20% heel, and then you use the main to loosen or tighten in real time. I only change the furl after that when the weather actually changes.
6) If you're not using a roller furling Genoa, go get one

I'm really not sure how I'd balance the helm with a hank-on jib with this boat.