Officers said "interference with signs" increased delays on roads around Weymouth on Sunday.
Drivers reported taking three hours to make a four-mile journey between Weymouth and Dorchester.
Organisers said despite advance warning, people had not fully understood the impact of the event.
Motorist Caroline Cooper, who said she arrived two hours late for a christening, wrote on social media: "It was a complete shambles. No diversion signs, no warning signs and nobody knew how to get anywhere."
Tim Brown said: "At no point prior was there any notice saying the road would be closed on Sunday."
Dorset Police said it would discuss feedback with the organisers to avoid issues during future events.
Insp Pete Browning said: "Unfortunately, there have been a number of incidents of interference with road signs. This irresponsible behaviour has exacerbated some of the traffic problems."
Peter Finney, Dorset County Council's Cabinet member for environment, infrastructure and highways, said a meeting would be held to discuss "all the positive and negative aspects of the event".
"We are aware of a number of issues with the traffic management for the event, including signage and marshalling. We will make sure lessons are learned and the necessary improvements are made to for any future events to keep inconvenience to a minimum," he added
Weymouth Ironman director Alan Rose said diversion signs on the perimeter of the event had been taken away or moved.
He added that more than 70,000 letters had been sent to warn people who lived on the 56-mile course.
But he said some drivers had not understood the likely impact of "biggest Ironman event the UK has ever hosted", involving 2,200 competitors.
Ironman, which takes place simultaneously at sites around the world, was previously held in Sherborne between 2003 and 2008.
Police have condemned vandals who removed road signs and helped to cause traffic disruption at the UK's "biggest-ever" Ironman event.