Are torrents less efficient than regular downloads?

Torrents work by trading a few extra bytes downloaded and some extra software complexity, CPU time, upload bandwidth, and additional active sessions in exchange for the potential to download the desired bytes at a much faster rate.

So it depends on how you define efficiency. If your primary concern is for transmission size, then torrents are not efficient. If your concern is how fast you get your data, they are great.

Each of the costs I listed in my first sentence are real. They are designed to not be a factor for most users, but any of them can be important in the wrong situation. For example, the small college where I'm network admin has a fixed limit on the number of simultaneous sessions we can push through our gateway at one time. BitTorrent users can generate in excess of a thousand sessions each. Just a few users like that and suddenly internet access suffers for everyone even though we have bandwidth to spare, because we can't process more sessions.