So your assertion is that if you bumped up the CouchDB connection limit to 1,000 (believe it or not, Erlang--in addition to Node--is capable of this), then this slowdown would not have occurred?
My point is the thing you are connecting in the back-end has to scale at the same level as node - if not better. I'm sure making independent non-pipelined requests to CouchDB will help, but if CouchDB views are slow or it can't handle the concurrency, then the web requests to node.js will start slowing down resulting in a pipeline stall.