I don't believe online is evidently faster or slower than mail filing. Each case is unique. Online or mail filing is only one variable in the process. We don't know who gets assigned the case and how busy they are, how quickly they work etc etc. Unfortunately USCIS does not publish N-400 statistics for each type of filing.
While mail can get lost, don't forget how glitchy USCIS online systems are. I'm not surprised if a anything uploaded / submitted can get corrupt or lost in USCIS online system. Again, there's no proof of upload that you get as far as I know. If USCIS says you never replied to their RFE online, how would you prove you did? With mail, at least you can have certified mail.
I'm not saying online or mail is better. Either has advantages and disadvantages.