Performance of internal combustion engines varies greatly depending on altitude due to thinner air. This is even worse on hot days. That's not the case with an

Performance of internal combustion engines varies greatly depending on altitude due to thinner air. This is even worse on hot days. That's not the case with an electric motor. As you get higher, you need to push more air, but since the air is thinner, it's easier to push. Most likely and electric motor with a variable pitch propeller would be extremely efficient. Electric planes would get all the benefits of high altitude flight (namely thin air means less friction) and none of the downsides. As someone else mentioned, if the batteries get too cold you run into other issues.