If you’re looking to save the last few seconds and know with absolute certainty that there’s no more time to be gained elsewhere, then wear a tri-suit. But for most athletes there are pros and cons for each type of kit and, ultimately, the choice isn’t time critical.
In a long-distance triathlon comfort is a higher priority than speed, especially if you’re new to the sport. It might take you 2mins to change from your bike kit into your run shorts and vest, but you can easily lose that by walking for a couple of kilometres later in the run. So you might as well take the time to change so you don’t have to deal with a chamois in the latter stages of the run.
To be certain your intended wardrobe is right for you, it’s a good idea to test your preferred choice several weeks before the race. You should also practise your transitions regularly – especially if you’re planning on changing outfits. Being slick through the transitions will also save you more time than you might lose by having two changes of kit.