I imagine that communication within range of these relay satellites is virtually instantaneous, as seen in Enterprise
and recently in Into Darkness
(where Kirk phones Scotty on Earth from the Klingon border). But when communication between parts of this satellite network is disrupted (which could mean anything from some space anomaly to damage to malfunction) or a ship is out of range, the signals have to be sent the slower, non-boosted way which is what we see in all the episodes and movies where communication takes days or weeks to reach HQ.
In other words, long-distance subspace communication from a ship to a distant target is slow, but when connected to the satellite network it's virtually instant.