Mood: Out with the grey....in with the satin black
Yes, there are big differences between the groups. As it's your 1st trackday, book yourself in the Novice group until you're comfortable with track riding.
There's a DSC trackday at Brands Indy this Wednesday
pure speculation, but at the indy circuit, approx 5 second spread across each group, with possibly some overlap between groups, and novice having a broader spread of times.
e.g.
Novice 60-75 seconds / lap Inters 60-70 Fast 50-60