Does the hub have an external power supply?  The USB miners are each likely to draw rather more power than can be supplied by USB 2.0 without being propped up, resulting in inadequate performance, premature failure of the USB built-in power source, or both.

I have found that the internet connection has the most effect on my hash rate; a fibre-optic connection is much faster than ADSL, which tends to have a lower rate at any time of day.  The hash rate falls away at peak usage times, particularly when the little kiddies arrive home from school and log in, and the bigger kiddies get home from work and start playing WoW.  In each case the difference is significant.

The hash rate reported by Slush is an estimate, based on work reported, and can take a while to stabilise after any interruption.  The true hash rate per device should be what you see in the BFGminer window.