[Good Problem to Have?] General Contractor going FAST

As a tangent to the other GC thread I created today, has anyone ever used a GC that worked extremely efficiently as opposed to very slowly?

Everything I read online makes it sound like you should double the time estimate they quote you for a job, but I've had the opposite experience on my current (and first) project. I figured the GC would quote me one month to finish the job and it would actually take him two months, but instead he quoted me one month and it looks like he'll be finished in three weeks or less!

Are there any warning signs I should look out for in such a quick job?

It's a full house (1,200 sq foot) renovation, including complete demo of the kitchen and master bath, new floors throughout, new interior and exterior paint, patching up drywall from previously done electrical and plumbing work, new tile, re-finishing existing wood floors, installing new doors and trim, and more.