Questions about wetsuits

Whether or not it's worth it to switch to a wetsuit depends on where you swim. Are most of your races wetsuit legal? If so, it's up to you. Wetsuits tend to help mediocre swimmers more than very good swimmers. But even good swimmers will have better times with one.

It shouldn't take you more than 10-15 seconds to get the wetsuit off. From the water to T1, you can remove the upper half and strip it to your waist. In T1, you peel it down below your knees and then just step on it to get your feet out. Plenty of Body Glide ahead of time makes this easier. You can even be pulling your helmet on while you get your feet out of the suit.

As for why you don't finish the race in it? Well, most people don't feel like getting pulled off the course due to heat stroke. Seriously, being in the water with a wetsuit on is one thing. But out of the water and doing any kind of exertion, you're going to overheat in a matter of minutes.