Obviously, mere distance isn't a hindrance, as in "Unimatrix Zero" extremely distant Borg units could be contacted in real time.
However, there's probably some sort of software hindrance to constant and complete sharing of thoughts across the entire Collective, as even the Queen seems to suffer from a slight delay in accessing specific pieces of information. (Or perhaps the Queen is handicapped in this respect, as she is designed for a degree of individuality?)
There certainly exists the option of cutting off specific parts of the Collective temporarily or permanently, for protection or for detached missions. The humblest Drones seem to come well provisioned for such situations, only very slowly succumbing to "homesickness", and usually being shown effortlessly readapting to the Collective when communications are re-established. Since the Drones manage well in emergencies, one might suspect they could also practice some sort of semi-isolation during the normal course of matters.
But the connection isn't a simple, homogeneous setup. While a bunch of Drones can link up with each other without need for external hardware, we know the individual ships have the Vinculum device of "Infinite Regress" fame to facilitate the shipwide connection. There may be other hierarchial levels above and below that, too, and multiple backups for all sorts of contingencies.
Timo Saloniemi