Basically what Steam does on big games is you download them compressed, and decompressing uses wayy too much CPU, and to not fully render your PC unusable Steam only uses half the CPU cores for.
Regular download without need to decompress anything barely uses any CPU (just disk obviously).

This causes download speed to be a lot less than what its supposed to be on games Steam does this on than they are supposed to be, being limited by CPU.

Because of this stupid decision by some incapable monkeys at Valve i get constantly way slower download speeds on Steam than on Origin, Blizzard, Uplay etc... while having 50-60% more CPU usage than the others slowing down my PC.

People with like 50 mbit far away from civilization with normal internet speeds can still have their compressed downloads since it is probably the more optimal way for them, just have it as an option in settings.