USPS Slower than First Class Mail Internet

My complaint is regarding priority mail flat rate envelope service (specifically from Western and North Central Kansas to Jackson Michigan). 

On Friday 12/9 I sent a priority mail flat rate envelope from the Post Office in Oberlin, Ks to Jackson, MI...I paid the additional .70 for delivery confirmation...expected delivery was Mon. 12/12.  Monday came and went, Tues came and went...envelope was not received.  Checking online, calling USPS and calling the actual post office it was mailed from would only confirm that the envelope was dispatched to the sort facility in Oberlin on 12/10 at 11:30am, no scanning after that.  The envelope was finally received in Jackson, MI on Thursday 12/15 (where it was scanned at 4:20am).  I had the same experience with a priority mail flat rate envelope mailed from the Post Office in Tribune, Ks on Fri 12/23.  Expected Delivery was Tues. 12/27 - again nothing received Tues or Wed, and again the only tracking information available was that the envelope was dispatched to the sort facility in Tribune on Fri 12/23 at 3:24pm...no other scanning occured until it was received in Jackson MI on Thurs 12/29 at 4:24am.

The same location that I was sending this correspondence to sent me a letter on Wed 12/22 via first class mail and I received it on Sat 12/24.  How can it be that first class mail can be received in 2-3 business days but priority mail takes 5 business days?!

I would really like to know what route mail takes from Western and North Central Kansas...it is the only place I experience these types of problems.  Something needs to change!