It's definitely a concern, but I think it's mitigated somewhat by the oplog-tailing approach. Writes go straight into Mongo, as usual, it's just reads from the oplog that potentially tie up resources on the webserver.

Those reads are throttled, so I think the net result of too many writes is just a choked up webserver, and slow "realtime". Potentially a future architecture moves the oplog processor onto it's own server and the effect is isolated to the realtime part.