Fix gear is significantly more effort and more physically draining though not much more of a workout and at times less. Effort is a huge part of fix gear riding. Typically fix gear rides require between 25% and 33% more effort than a geared bike for the same ride. Now - effort cannot be measured on a machine. It is not watts, calories expended or anything else you can document. But all of us who have ridden the road long enough both fixed and geared know what I am talking about. (Lifetime I've done roughly 100,000 miles of each. I've done Cycle Oregon twice geared, four times fixed.) Fixed is a LOT harder, even when I bring all my cogs and can ride any gear I want (but have to both plan ahead and stop to change gears, much as they did 100 years ago). When I do Cycle Oregon geared I can stay up and listen to the music (which ends by 10pm). On the rides I do fixed, it is hard just to stay up late enough to catch the announcements re: the next days ride. Going into town after setting up my tent doesn't happen.
Riding fix gear is also wonderful for teaching good pedaling skills and recovery while pedaling. Going fast fixed downhill is all about teaching your muscles to relax completely when they aren't actually powering you. Downhill they aren't. So the better that part of your pedaling, the faster you can go downhill. Two years after I finished racing and most of my rides were on my fix gear, I used to do crazy rides up Oakland's Jauquim Miller, turn around at Skyline Blvd and bomb down the 1000' back to Oakland. I never saw my speed, but I was never passed on the 4 lane parkway with median that was usually driven at 50. All on a 42-17. So fast on small gears is quite possible if you train hard enough to relax those muscles, have the hill and have an attitude of either suicidal or crazy. (My post head-injury "crazy years. Those rides kept me out of institutions and off substances.)
Yesterday I rode 70 miles with a hilly circle around a reservoir. Today I rode the same bike in town and back. I had no legs so rode the 42-17 instead of yesterday's 16 but I was even faster downhill. Exactly what I was talking about above.
Ben