The same day I had my biometric appointment I had an interview (Oct 1) and I didn't want to miss it or change the day, the interview is in the morning and the Biometrics appointment was at 1 PM, but I didn't want to take the risk to miss the Biometrics appointment, so last night I started googling if there was any way to speed up the process, and I found out that some USCIS offices are able to take your Biometrics, as long as you have the appointment letter, even if you are going before... well, I went today Saturday to the USCIS office just to try and knowing they might have said no... guess what?... They were totally free and so nice! They took my Biometrics and now I don't have to wait until Oct 1!. They were so nice!, so If you are in a hurry you can try going to the Biometrics USCIS office before the appointment day! I read on other threads that the best days to go are Wednesdays and Saturday; they are less busy those days(That's what I read, I might be wrong). However, it depends of the office. I was lucky!, but they could've said I had to wait.... anyway, I'm so happy! You should try if you are in a hurry! It's is worth to try if you really need the papers fast.
I'm just telling you my case, I'm not a lawyer or an expert, so act accordingly.