there is lots of solid data on this - some is quite weighty.
Here is a short version
the lower the effective density - the more lift is needed.
The easy proof is in using an electric motor powered model
At sea level- you can set a base line for watts required to fly in a fixed manner.
Now get in a balloon and launch at various higher altitudes.
Read how many watts are used to do the same tests as you go higher .
The big difference will be the changes needed in the prop used (increased pitch and diameter ) to draw the same watts.
You will need to go faster to get same lift and if you can "hover" you will see the prop requirements change as the air gets thinner ( more pitch and diameter to hold the same watt loading.
The plane can go faster the higher you get as there is less density to swim thru -. On an electric, the motor (power source) is constant at any altitude - so the result will be easy to see.
you can go faster on the same power .
On a gas powered model - the power decreases faster than the reduction in drag so you can't get a valid comparison.