fork+exec is even more expensive than fork. (Although in the case of video encoding the overhead is negligible. Problems with Node.js are more likely to be seen when an event sometime takes 100-1000 ms; it's slow enough to hurt response times of other requests but maybe not worth farming out to another process.)
now writing a high-throughput server is simply a matter of writing a high-throughput worker to generate fibonacci numbers and connecting it reliably through interprocess communication to your node.js shim.
thank god node.js was there to save me all that work.