Crew and maintenance are both higher costs for most airlines, according to this and other sources I've found: https://www.icao.int/mid/documents/2017/aviation%20data%20an...

Let's say that you can now make twice as many flights in a day. You can now do twice as many flights with the same crew and aircraft (same maintenance costs). Your fuel costs increase, but your other costs remain fixed, so despite the increase in fuel costs, you still end up making more profit. You might be able to charge more for the faster flights as well, and there may be beneficial second-order effects.