As @ramez stated, it will definitely be less CPU intensive out-of-the-box, because you won't have to tail the oplog, which often results in CPU spikes if there's a lot of data comming through.
If reactivity is mostly focused on userId, meaning I, as a logged-in user, will only see reactivity for elements where userId is me. Then what you need to use is "namespaces".
Channels are used for stuff like messaging, where you chat in a thread, and you don't want to watch the reactivity of all the "messages" being sent.
Documentation is on it's way, so I would clear out many things and what to use.