It probably has to do with how SolidWorks handles sketch constraints. As you add sketch elements, the number of sketch constraints required to solve increases multiplicatively. Thus you always want to keep your number of sketch elements, and more specifically the number of sketch constraints, down to a minimum.
A good way around this is to use multiple sketches. Create a base sketch, then close out of that sketch, and create a new sketch on the same plane. You can use the convert entities tool to bring existing elements in, as this constraint does not require solving. When I create very complex sketches, I often create two or more sketches. There is the base sketch that defines the foundational geometry, then the final sketch (used in the feature) that contains all the small and transitional elements.