"[...] is because of asynchronous I/O" (too many isisisis).

actually, non-blocking asynchronous I/O is possible because of the background threads. so the point about threads belongs in the main body of text before the comma. maybe something like:

> Node uses 4 (by default) threads in the background to perform non-blocking asynchronous I/O.

though I feel like you could put together some better wording.

also I'd drop the node is fast bit. it's subjective and imho doesn't belong in the api docs.