Like Solidworks, a sketch doesn't have to be fully constrained, the method allows for more flexibility and freedom in design, there's also an option to have the sketch "fixed" if you don't want it to change. Depending on the user, I used all three, and to me I think proE is a bit complicated...
Your assembly is considered small to slow down the computer, there nust be some thing else involved. You might want to try having the Task Manager open while working to see which software is using the resources.