I was going along at about the same pace, with a bit of a stumble adding decimals (I claim because the numbers a aligned funny), but "Multiplication 4" threw me for a loop. I kept missing questions and had to restart my 10 problem streak. I made a number of mistakes, and I discovered a number of tricks, but a few of them were noteworthy.
First, when I was using the mouse to draw numbers I was much slower at thinking and drawing. I am not accustomed to using a mouse, and when I write I use my left hand, so I think these two factors played a role in slowing me down.
Second, when I finally got home from school and figured I should get serious and get past this exercise, I pulled out a small pad and started doing each problem, twice, on paper in front of the lap top. To my dismay I answered the first question wrong. The second question seem to take me about 10 minutes.
I think there is something to be said about warming up. Earlier in the day I was slow, but I never missed 2 questions in a row. I think using the paper was a benefit, but I think taking the evening off and then returning to the problems slowed me down and I didn't have the benefit of some easy warm ups (by which I mean the earliest "Addition" and "Subtraction" nodes I completed prior to reaching "Multiplication 4").
I came up with 2 ideas for this experiment. One is to return daily to "Addition 1" and complete 10-20 problems. I will have to do them carefully, because I want to get a streak in the thousands. I feel like this will gather some sort of attention somewhere in the Khan offices, but maybe not.
Second, I want to return to "Multiplication 4" sometime in the future for a similar sort of workout. This time, though, I want to warm up with some easy addition problems and see how I do. Obviously when returning I will have the benefit of having my daily practice to warm me up, but still, I would like to consider how the warm up affects my performance.