DEF sucks not because it is expensive (it is not). DEF sucks because it is mandated by the EPA and is otherwise not needed. In fact, the truck runs just fine without it so .gov made the manufacturers institute all sorts of intrusive mechanisms to force you to refil it.
If it runs out it will limit your speed.
If the fluid is poor, it will limit your speed.
If a sensor is bad, it will limit your speed.
This is why DEF sucks.
