There's no straightforward answer because there are many factors that affect flight time and fuel economy, including the aerodynamics of the plane and the engine technology. I hazard a guess that for commercial airplanes these are chosen primarily for reasons of fuel economy per seat and then that determines the model's designated cruising altitude.

For a particular model, flying above the model's cruising altitude should lead to lower fuel efficiency.

From what I've learned reading AdmiralCloudberg's plane crashes analysis [1]: altitude heavily matters in fuel consumption. Jet planes use a lot less fuel at a higher altitude, up to the point that a plane on the verge of running out of fuel at a medium altitude might manage to squeeze in 50 or 100 more miles of flight by climbing 5000 feet, even accounting for the increased fuel consumption during climb. I guess that correlates with speed as well. Turbofan engines, on the other hand, are more fuel efficient than jet engines at lower altitudes, hence they remain common for interstate transit. The difference seems to be directly caused by the effect of air "thickness" on the engines.

[1] https://admiralcloudberg.medium.com/

I think in a way that’s why planes fly so much further up than you’d think they’d need to. They want more consistent and minimal atmospheric conditions. Less air means less energy means less turbulence, I think?

If you’re talking about friction… oooh that’s an interesting one. Intuitively yes. But is it also negligible?

Yes, because friction depends on the air density. You can think of this as the molecules in the air colliding with a moving object and pushing it backwards, thus slowing it. If there are fewer molecules, there is less friction and the object can move faster with the same thrust.

Thrust itself decreases because there are also fewer molecules to push against, so it can get quite complicated if you want to account for everything. But overall it is easier to fly faster higher up in the atmosphere. Also, atmospheric currents are important.

There is a useful discussion here: https://aviation.stackexchange.com/questions/57209/how-does-...

Presumably this would affect drag significantly. Here are the equations of motion of an aircraft https://eaglepubs.erau.edu/introductiontoaerospaceflightvehi...

...and indeed it does. Here is a discussion https://aviation.stackexchange.com/questions/24641/what-is-t...