From experience driving to town everyday from Pt Adelaide in peak hour traffic and coming back home in the same traffic, i found that it only took me on average 25 minutes.All the traffic lights become predictable and you will rarely ever get straight green lights. Overall its flow is fairly good and like Tom said once you pass South Road you encounter a bit of a traffic build up.
Port Road usually flows fairy well even in peak. You will experience some delays in the mornings, especially from about South Rd, but compared to the other side of the city, they are minor. Outside of peak hour, no issues at all.