Speedy Checkout lets you book and pay for places without entering everybody's details at the point of purchase. Not every event offers it.
It is worth using if you do not have everyone's details to hand, you are on a mobile, or you are simply short of time.
What you still have to do
You are paying for places, not completing entries. You must come back to Race Space and register all participant details before event entries close. You will get email reminders until you do.
Why the deadline matters
If registration is not complete before entries close, you may not be able to take part. The place is paid for but the entry is not finished, and no ticket is issued until it is.
If you bought places for other people, you do not have to fill in their details yourself. You can send the entries to them so they complete their own.