When something takes 10 days to deliver, most people say it will take only 1 week, to come over quicker and better.
The opposite actually happens in the mind of the client . You will seem sloppy and unproffesional because.
If you'd say it it will take 2 weeks, and you then finish it in 10 days, the client will love you way more. You mentally prepared them for something long, they accepted it, and then you finished it sooner, amazing.
Now you're doing better then you promised them, while you're doing nothing differently. You just packaged yourself differently.