"oh, that's less than a normal download, like from Steam, so it must not take nearly as many resources"

For me it's always more.

The amount of encryption and hashing involved in torrenting is fairly CPU heavy

Same amount of encryption https requires. Hashing is completely optional and is not required for operation, encription is optional too, but other peers may require it.

every ~4 MB piece has to be hashed and verified

Which is same. I'm not sure, but steam probably verifies file at some stage too.

The sheer number of connections involved even in just one torrent can also bog down a network like you wouldn't believe -- anyone who runs a home seedbox can attest.

There is not difference between 4kpkts for 500 connections vs 4kpkts for 1 connection for network itself. IP and UDP don't even have such concept. Network is stateless. But shitty routers with small conntrack table on the other hand...