So computers get turned off every night and started every morning? If you're using the default settings and clients are updating every day, then they should only be requesting delta packages for definitions (the difference between what's available and what they have). Thirty minutes is definitely way too long.
One thing to try if you haven't already is to adjust the download randomization to something higher. This is found under the Communication settings for the group (Clients > [select group] > Policies tab > Location-independent Policies and Settings > Settings > Communication Settings... the same place where the heartbeat interval is set), and the default is also 5 minutes.
Do you use AD startup scripts? For some reason this comes to mind (don't let the title fool you--I've seen instances in which slow start-up was resolved by this):
Title: 'The Windows Explorer Shell (explorer.exe) fails to load at logon when Symantec Endpoint Protection (SEP) is installed'
http://service1.symantec.com/SUPPORT/ent-security.nsf/docid/2008040712253648
sandra