As someone who is forced to travel the A27 towards Southampton often, I'm sure that most of the delays delays are caused because roundabouts and sets of traffic lights mean that the main through route doesn't get priority over minor roads. I may be wrong but it seems to me that junction redesign...
I came to the conclusion a while ago that it's far quicker to drive straight through the middle of Chichester than waste time crawling round the bypass. And going cross-country via Steyning and Storrington avoids both Worthing and Arundel.