It looks like you are using only 1 perimeter/shell. This is incorrect, please use:
Top shells: 4
Bottom shells: 4
Outline/Perimeter shells: 3
The 15% infill (my profile default) does not really matter then, because with 3 perimeter shells, there aren't really that many gaps left to fill in any more. The perimeter shell is important to get the springiness, because it makes the nozzle follow the outline rather than do a criss-cross pattern.