B737 Undifferentiated or Other Model
Flight Crew Air Transport Pilot (ATP)
Flight Crew Type 3742
Situational Awareness; Confusion; Communication Breakdown
ATC Issue All Types; Deviation - Speed All Types; Deviation / Discrepancy - Procedural Other / Unknown
Flight Crew Requested ATC Assistance / Clarification
Company Policy; Aircraft; Human Factors
Our flight departed six minutes ahead of schedule; despite low ceilings and high winds. Fuel burn and time over the fixes were 'ahead' as we proceeded at our planned Economic Cruise and in contact with Minneapolis Center. Center instructed us to turn left approximately 60 degrees and to reduce our speed to Mach .74.  I found the clearance very odd. An unidentified aircraft traveling behind us at 38000 feet had been slowly overtaking us. We complied with ATC instructions and were cleared back on course. I requested 'normal speed' and was informed that we could expect to remain at Mach .74 all the way to our destination.I then asked why and was told that it was due to sequencing and high traffic demand despite clear weather and winds from the south that would present no problems to a normal 'south' airport landing configuration. I then asked ATC if the vector and the speed reduction was to accommodate the aircraft at 38000 feet. A second voice; female; came on the radio and informed me that they 'could not provide that information.' I requested a telephone number with which to reach the ATC facility. Initially; the female controller refused to provide me a telephone number but; after my second request; gave it to me.We landed and blocked in.  Cruise at the slower Mach number; approximately Mach .04 below our optimum speed; resulted in an 'over-burn' of 800 pounds of fuel versus our planned fuel. Planned fuel indicated 9;500 pounds remaining; actual was 8;700 pounds. There was no traffic to speak of at the destination; visibility was greater than 10 statute miles; and they were landing via triple visual approaches. We executed a Visual Approach to without incident. I called the Center and spoke with a supervisor who said that he would complete an MOR (Mandatory Incident Report) in response to my concerns.I believe that Center needlessly compromised the overall safety of my flight in an effort to accommodate the other aircraft at 38000 feet and I believe that the female controller's impatient and hostile response to my request for information on a relatively quiet frequency was a clear indicator that she was uncomfortable being 'caught' doing it. Is it reasonable to burn through 8.4% of my estimated fuel remaining for no apparent reason? Where on our 'Fuel Ladder' is there a line to account for this? Had my destination been IMC and had we needed to hold; that squandered fuel represented over 13 minutes of holding. ATC irregularities like this are unsafe and immoral. Intentionally economically disadvantaging one airline to the benefit of another is illegal.
B737 Captain reported they were unnecessarily vectored off course and assigned a slow speed to allow other traffic to pass them.