For CouchPotatoServer (AKA CouchPotato 2), I would suggest having SABnzbd extract the final movie to a location not on the network (Droboshare) i.e extarct to a local drive.
Make sure that your location for "temporary download folder" in Config->Folders is definately set to a local drive.
Then configure CouchPotato renamer to do the move (set in the renamer page) from the completed movie folder to the final (Droboshare) location. This way you can be sure all renaming and extracting is done on a local drive. If this still causes Droboshare to bog down, then the limitation is on your network/Droboshare and not in SABnzbd processing.
SickBeard can be configured the same way in case you run into issue with that.
Also, to avoid SickBeard and CouchPotato constantly scanning your network shares etc for completed media, you can set up postprocessing. Sickbeard comes with a built in "sabToSickBeard.py" script and I have created a "sabToCouchPotato.py" that can be found on the CouchPotato forums http://couchpota.to/forum/showthread.php?tid=343
, but the default renamer should do what you want.