I've definitely seen it referred to as IO bound when an upstream server just can't get your input fast enough, whether that's because you've maxed out the mediu

I've definitely seen it referred to as IO bound when an upstream server just can't get your input fast enough, whether that's because you've maxed out the medium (e.g. you're bound by a 10/100/1000 Mbit link), or bound by how fast the disks can serve up bits, to downstream it's generally encountered as the same thing, the ability to process IO faster than it's available.