[joecarpenter]

It is mostly number of documents and their size. Single schema.

There are no indexes, so whenever client wants to find something in synchronized collection, it'll have to scan through all documents. With 10k documents it was locking up DOM (it was very noticable).

If documents are large, client won't be able to connect to the server with any of the polling transports - initial synchronization time exceeds 30 seconds and SockJS will drop connection. This is partially solvable by having reverse-caching proxy (i.e. nginx) forcibly compress all polling SockJS responses.