i do not believe a conjecture can really slow down progress. (i believe that was the 5th postulate by the way.)
the stupidity of people in sticking to a conjecture's truth rather than exploring other possibilities may slow their own progress in deciding it, but this should not be blamed on the conjecture.
If you look into it, I believe many conjectures were actually just questions until they were solved, and afterwards their askers decided they had been conjectures.
other famous conjectures were actually mistakes, and when they were exposed thye became conjectures.
the only case I know of where progress was actually slowed, was not by a conjecture, but a false theorem, the case when pontrjagin announced an erroneous calcuation of a certain homotopy group.
his announcement contradicted bott's conjectural belief as to the periodicity of stable homotopy groups. pontrjagins fame and reputation, caused bott to delay work on his own correct conjecture for some time.
at length the error was revealed and bott immediately began his successful proof.