This is what I see tacking, in the test build in the world scenario (17kt wind):
I start with the wind 1pt free, going at 7kts. I order the helm hard over, and ease the headsails and take in [edit: haul in] the spanker once I am clearly turning. I am pointing at the wind after 60s, and still going at 1 or 2 knots.
If I start 1/2 a point closer to the wind, going at 5kts, then this takes 120s, and for the last minute I am nearly motionless or moving backwards, being pushed around just by the foresails.
Maybe this is as it should be - to tack snappily, you need to build up some speed. I haven't tried it in a lighter wind.
I also observe the following. Start the world scenario, set the waves to 0 and bring the wind right on the beam, with physics time acceleration on high. You will reach 9.7kts. Take in all sail abaft the mainmast, and you slow down to 9.3kts.
So far so good. But now do the same thing, starting one point closer to the wind. You will reach 7.1kts. Take in all sail abaft the mainmast, and you speed up to 7.3kts.
What is happening is that with the sails set in the second case you need one spoke of weather helm to balance them, and the drag caused by the rudder slows you down. With the sails taken in, you have a tiny bit of lee helm, and essentially no drag from the rudder. This is deliberate behaviour, sort of, because I wanted drag on the rudder to be a consideration in your sail plan. Possibly it is too strong right now - but I'm not sure how to tell. Reducing drag from the rudder would also make tacking a little bit easier.
Edit: Substantially decreasing the drag from the rudder doesn't change this, so my explanation above is wrong. It may be something to do with the extra leeway, with more sails set, causing more drag; but that doesn't work by itself, because removing sails forward would also decrease the leeway. It should be the torque from the sails that is the problem.