Boogie Nights is one of the most creatively unique movies of all time. Yes, it drags towards the end during Marky's downward spiral into drugs but for most of the film it is a marvel of capturing the desperation and self delusion of a small and wacked subset of the entertainment industry. Marky Mark and John Reilly running around as secret agent wannabees was some of the best movie schtick evah.
Also, I think the change in pacing near the end was intentional. The first part of the movie was fast moving, emblematic of Dirk's lifestyle. Then when he was getting dragged down the pacing slowed and was uncomfortable so that the viewer, in a way, was feeling the same thing as the character.