wireless & bandwidth question

I have a situation with an installation that is using a Sonicwall TZ180 Wireless router.  They have 2 wireless connections, one secure, one open from within the same router.  Te secure connection is terribly slow and loses connection often.  The open one is fast and stable. 

Is the open connection sapping bandwidth from the secure one?  Are the channels too close?  DHCP enabled?

I haven't a chance to look at any configs yet,  but any ideas on what to look for would be greatly appreciated.

Thanks