There are lots of great tips in these posts.
The contact of the ski with the snow is the greatest contributor to maintaining, or not losing, speed:
- good wax and base preparation,
- smooth, well prepared edges,
- proper base structure,
- as little edge use as possible,
- bring speed into the flats,
- reduce any turning on the flats to the bare necessity to make the gates,
- roll gently onto and off of the edges,
- steady even pressure instead of hitting the edges hard
Notice that many of these tips involve edging skills. Edges are slow, bases are fast. The least amount time and the least amount of pressure spent on the edges, the better. The putting the weight back, not forward, is accurate, but hard to teach a budding racer. She may forget to get forward when needed.
Good aerodynamics are important as well. Create the smallest profile to the wind as possible. Lead with the hands to create a longer, more aerodynamic object. Chest down, hands in front of the face so you can just see over the hands. If you need to come up from a full tuck to a high tuck, keep the chest at the same height as the chest to avoid creating a pocket for the wind to get trapped in. Move the hands further forward and together in front of the face. Avoid letting the arms go to the sides.
My under-informed guess is that on the flats and straights your little racer isn't as flat on her skis as she might be. It isn't uncommon to get as low as possible with the feet a good distance apart but with the knees too close together, essentially skiing on both inside edges. A good flat ski position while tucking isn't particularly comfortable as the knees have to be over the skis for the skis to be perfectly flat. It takes a conscious effort to focus on a flat ski.
Practice tucking outside of the course, hopefully with a slightly soft surface on the snow that will show the tracks of the skis. Until the skis can run totally flat, they will be on edge and slower than a flat ski.
Not knowing the course or seeing your racer makes it difficult to give specific advise. Please supply video or photos if possible. Getting faster on the flats is a difficult endeavor.
One place that it easy to pick up time on a course is at the start. While it won't prevent a slowdown on the flats, it will provide a cushion of time that will reduce the impact of the flats. Use a good strong kick, followed by long hard strong pole strokes and skating motions. Avoid short arm and leg motions. Look like a Nordic skate skier out of the start. You can pick up .5 to 1 second by improving a start. The beauty of starts is that they will help with every course, not just the ones with flats.
Best of luck to your daughter.