A fast track package is a quick contest which runs for 4 days. In the fast track package, each designer is allowed to submit max 4 designs during design submission stage. Whereas in standard package, there is limit of max 10 designs.
Fast track package is created especially for customers who want a quick turnaround without compromising on quality.
But in both packages the limit is extendable to unlimited designs at the discretion of the client. Every time a contest holder gives a star rating of 4 or 5 stars on a design, designer's design limit will be increased by 1. So if you give 4 or 5 star rating on a designer's 3 designs, their limit will increase by 3.
There is a small quality difference, but not much as by limiting the design submissions, the designers only submit quality designs.
You can check various packages here.