Swimming in a wetsuit on its own should result in faster swim speeds, by nature of the extra buoyancy you gain from the neoprene. This lifts you up in the water, and weaker swimmers will notice the benefit more so than stronger swimmers purely because their legs will tend to drag a little lower in the water.
For someone swimming 5mins per 400 metres I’d expect that difference to be up to 5secs/100m quicker, for others the difference may be even greater. One potential issue is that your swim may be quite kick-driven, which could result in your feet actually airing rather than propelling you through the water. This means you’ll lose some of the propulsion you would otherwise gain (given that air is much less dense than water), so it’s worth having a feel for where your legs are in relation to the surface – or you could get someone to watch you swim and point out if you’re creating a lot of splash through the water.
Another thing to think about is wetsuit fit. Different brands will fit quite differently. As a result, it doesn’t necessarily matter how pricey your suit is, if it’s restrictive across the shoulders because the chest is too narrow for you, this will definitely reduce your power output through the water and add to fatigue. Also, it’s worth bearing in mind when buying a wetsuit that the higher-end models will usually be more flexible, and you’re going to get much more freedom of movement in a £400 suit than a £150 entry-level suit. Having said this, I think fit is far more important than price, and you’ll find an ill-fitting high-end suit much more restrictive than a cheaper one that fits you perfectly.
Finally, how are you measuring the speed of your OW swims? Are you swimming in a straight line? Don’t forget the more you lift your head to sight, the more resistance you’ll add (as your legs will sink slightly). Focus on making sure your stroke is balanced (left to right) and that your core is straight and strong at slower speeds. Also, make sure you don’t lose that balance as you speed up. It’s easy to hold a straight line in the pool without thinking about it, but if you’re constantly having to correct direction it will naturally slow you down, so try your best to get this corrected.