I wouldn't think it would be 20% slower; that would be going from 850km/h down to 680km/h. The big problem isn't so much that it is much slower but in fact the

I wouldn't think it would be 20% slower; that would be going from 850km/h down to 680km/h. The big problem isn't so much that it is much slower but in fact the slower speed meaning a higher angle of attack which leads to more drag. At altitude commercial jet liners fly at a set angle of attack to reduce drag, so maybe flying a few percent slower might lead to less fuel usage, but more than that and the increased drag eats away at the savings.