some possibilities:

Your new sail seems to be about 80 square feet, so that can't be the problem!

What about the runners? Some problems with runners can be:

1) The rear runners, on the crossplank, MUST be aligned parallel to each other. If they are not, they will "fight" each other while the boat moves forward, and actually act as a brake! Bring a good tape measure, or long stick, with you, and turn the boat on it's side. Measure the distance between the front of both rear runners, then the back. Loosen whatever mount they use, and turn them until they are as perfectly parallel as you can get them.

2) Runner edge angle: If the runners have too sharp an angle cut for the edge, then they will cut into soft ice and slow you down. Runner angles start at about 45 degrees, and go up to about 55, I think.

3) Runner length: A common problem with a home-made boats is that the runners are far too short. I've seen them with ones as short as 9" on the ice! But you should have over a foot, at least... and I think DN runners have 18" on the ice. Too short a runner will not allow the runner to "float"... it will cut a groove, and slow you down. You will also lack steering control, should you get moving.

I hope these help. Let us know how it goes... Rich.