Less secure circuit construction makes it weaker for anonymity protection, but drastically faster.

As the website states, don't rely on this for anything but downloading your favourite linux distros behind a censored connection.

What is the advantage of speed given the size of torrent files?

TPB no longer uses torrent files as their primary distribution mechanism. Torrent files were already small, but now they use magnet links which are basically just a SHA1 hash and a URL for a tracker. The tracker isn't strictly necessary.

Here's a fun trick: Make a plaintext file containing "Hello world" (maybe lowercase, maybe either will work). Take its SHA1 hash. Paste that into the Transmission "Open torrent address" dialog (edit: It looks like you can only directly paste hashes into the web dialog. You can still craft a magnet link manually). Give it like 5-10 minutes. Through the magic of DHT, you will soon have a brand new copy of the "Hello World" text in your downloads directory.

Magnet links are tiny, inconsequential even on dialup.