I've ridden with guys or DR's. The DR was 5-10kph faster on really tight dirt roads - BUT beat the riders to death to do that. If it opened up the DR's suddenly became chicanes.
Personally for long days including dirt roads a DL with good offroad tires is fine and probably better than a DR. The main point there is tires
Also, a comment, a DL with good tires was better in mud than a DR. I could just chug through where the DR's needed revs and used to get very untidy in the same conditions.
Don't get me wrong, there are places where DR's will absolutely thrash a DL, but that's not dirt >roads<.