It sounds high for sure. But they do serve videos, which possibly they have to transcode or otherwise process on import (?) They probably also have some filter

It sounds high for sure. But they do serve videos, which possibly they have to transcode or otherwise process on import (?) They probably also have some filters for pictures to remove porn and stuff like that. This can amount to serious CPU if you have millions of users uploading that stuff daily. A pure text-based Twitter would be much smaller.