Having observed rampant inconsistency in delivery rates with media mail both for personal and business use,  I decided to conduct a little experiment to see if how I mail a parcel affects how quickly it gets thru the system and to its recipient.  Long story short, what I have discovered is this:

If I mail with prepaid postage from the blue box outside or the mail slots inside, the package gets sent to the big city next to us that day and gets to the recipient within 4-7 business days.

If, however, I buy postage at the desk and hand over my parcel to them (same labeling with bar codes etc, just buying postage there), it has taken 7-10 days for it just to leave the same post office and get enroute to the recipient.

Has anyone else experienced this differential?

For mine the speed of delivery seems to depend on whether they go through Bell, CA or not. My outbound books sit there for days and days. I had one delivered last week that sat in Bell for 17 days, and then it went to IL instead of AL and had to be rerouted. Books that get scanned in San Diego instead of Bell arrive much faster.

The weird thing is that you can't predict which route they'll take by looking at the destination address. Two books mailed to the same east coast state one day apart are quite likely to take different routes getting there.

I did where I used to live.  I lived on an island for several years. All mail went off the island for processing on the mainland.  Even somethign that was just going to somewhere else on the island.  I noticed that if I put it in the blue box it arrived faster then if i mailed it inside the PO. I'm assuming that when they emptied the blue boxes that they just put it right on the truck to the mainland to go to the sorting facility. Whereas media mail inside the post office was probably held off if the truck was full.  Just my guess anyway.  Something similar could be happening where you are. 

Now I don't have a convenient blue box near me. I have to go to the PO, park and walk to the box. If I'm going to do that I might as well go inside and save myself the printed postage fee.