There are two premium services on offer:
1. The regular
premium service and you get a decision (for most cases) on the very day of your appointment. See more details
here
2. The
Super premium service that costs substantially more, £6000+premium fee at the time of this writing, and a decision is given within 24 hours. The difference here is that UKBA visits you at a location of your choice and your biometrics are taken on the same day. More information
here. However, with the super premium, applications from several applicants can be batched and processed together. The maximum is 4 applicants with a total of 10 people (main applicant + dependents).
I have experience using none, but the premium service is now quite common. Haven't heard from anyone using the super premium service (it's relatively new).