Much like the story Zodiac was retelling, it was long, drawn-out, and came to a less than satisfying end.
Though the 2 decade long serial killer story was condensed down to 157 minutes, much of it could have been cut (the pre-re-married Jake Gyllenhaal character with his son, all of the murder scenes, the scene at the theater owner’s house, Robert Downey Jr. on the house boat, etc) to put the focus on catching the murderer. There was plenty of interesting things going on at the papers, police stations, FBI, etc – all of it diffused by the character development stories.