My experience is that when I am riding fixed and I can hit the bottom of a hill with speed (esp in rolling hills) I have so much more invested in my energy and muscular resources that I pedal much harder while my RPMs are high than I ever do with a freewheel. As a result, I often climb hills much faster fixed. I pay for that later. I estimate fix gear rides are roughly 25-335 harder than geared and a real amount harder than SS for mostly that reason.
I do have to confess I haven't done a side by side comparison of FG vs SS for 39 years. I only did it once and it wasn't a good test as I never pushed it on the fix gear. That was my first fix gear ride. Never rode SS again. I was sold.
Ben