Modern JavaScript engines can compile the code to an intermediate form (or even to machine language) which may allow performance roughly equal with compiled languages like C.
While JavaScript is probably slower and use much more resources, it's not always the case.
I would also argue that bit-torrent doesn't take up much resources, at least when only seeding a few files, I guess you would have to seed thousands of files for performance to matter.
I know what javascript engines tout in their feature lists. I also know how javascript heavy websites/webapps behave. They don't inspire great confidence.
The IRC-like chat on Social Savannah will run up your browser memory and until you run out and crash. pdf.js (in firefox), as mentioned, starts sucking down cycles and bytes with just intel reference manuals open.
While JavaScript is probably slower and use much more resources, it's not always the case.
I would also argue that bit-torrent doesn't take up much resources, at least when only seeding a few files, I guess you would have to seed thousands of files for performance to matter.