It's not getting worse, she's just getting more efficient. Some babies are really fast feeders. Sounds like she's doing wonderfully!
It is confusing when your second child is different (I'm assuming your boy spent longer at the breast?) My DS fed for aaaaages, my DD was on and off within 10 minutes from birth. Now she's done in a few minutes.
As MM says, she must be doing ok with those weight gains. I think you can probably let her lead rather than trying to offer all the time, sounds like she's got the hang of feedign quite well. Just watch the nappies, they're your first indicator of a drop in supply. Your supply should be fine as long as she feeds according to her need - you make what she takes out.