The issue is in the title. For example...
100 acres

Smart oblique disabled is a 21 minute flight.

Smart oblique enabled is a 95 minute flight.

My takeoff speed is the same for both, but the speed with smart oblique enabled is not editable and it reads "6.1mph ~ 30.6mph"

I have not read of anyone else complaining about this so I'm wondering if I have a weird setting set somewhere or what is going on. It seems that this setting should reduce the flight time if anything, or be about the same.

Smart Oblique adds a 2nd flight plan that flys 90° off from the original, aka double grid. It also extends each flight line longer to catch the extra oblique shots needed for your original polygon area of interest. The reason speeds are such a wide range is because it's shooting at 3 different angles in majority of areas where it would normally only be shooting one. So the double grid would roughly double your flight time, plus the longer flight lines and the slower speeds would seem about right for the time difference you mentioned.

Smart Oblique consists of making 5 flight plans.  
The one at NADIR plus 4 others with an inclination and orientation North, South, East, West.  

1 flight plan 20 min, multiplied by 5, 100 min.  We can even say that at 91 min it is still optimized.  Everything is normal.

Johnnokomis Posted at 4-5 21:12
Smart Oblique adds a 2nd flight plan that flys 90° off from the original, aka double grid. It also extends each flight line longer to catch the extra oblique shots needed for your original polygon area of interest. The reason speeds are such a wide range is because it's shooting at 3 different angles in majority of areas where it would normally only be shooting one. So the double grid would roughly double your flight time, plus the longer flight lines and the slower speeds would seem about right for the time difference you mentioned.

Yes, you're right. The smart oblique I mention is that of the P4M.
The P1 and the M3E also have different modes, but the principle remains the same.

Nobody else complained about it because the "issue" you've reported it is a feature, not a bug. The "Smart" in the name should not be interpreted as "faster". It is supposed to mean "better" (which usually takes "longer").

Johnnokomis Posted at 4-5 21:12
Smart Oblique adds a 2nd flight plan that flys 90° off from the original, aka double grid. It also extends each flight line longer to catch the extra oblique shots needed for your original polygon area of interest. The reason speeds are such a wide range is because it's shooting at 3 different angles in majority of areas where it would normally only be shooting one. So the double grid would roughly double your flight time, plus the longer flight lines and the slower speeds would seem about right for the time difference you mentioned.

Makes sense, thank you!