Calibre not only has to move the data, depending on your settings calibre also has to explode/unzip the books an examine the contents. And perhaps convert and zip again. And it has to do this in a way that ensures that the calibre library doesn't loose data.
Calibre sometimes automatically does a lot of very heavy lifting, depending on the preferences set.
It is without any doubt the most demanding program I have installed on my computer, perhaps with the exception of Handbrake for video conversion. But Handbrake doesn't put a load on the I/O like calibre does, mainly cpu.