hokiefyd 

You'll get recommendations for narrower tires (like 28/30/32mm) and you'll get recommendations for wider tires (like 40/42/45mm) and both approaches have merit, and both will offer less rolling resistance than the stock LT2s (which have inexpensive construction and a relatively thick puncture protection layer). You want a tire, narrow or wide, that will return as much energy back to you as possible. This is a good video:


It's 35 minutes long, but I think it's worth the watch. They go into a number of topics, including finding the right pressure (not necessarily related to your question) and how different tire sizes and construction methods and materials influence rolling resistance (which is related to your question).

In short, buying supple tires with quality construction will reduce your rolling resistance and improve your speed. Science and observations are mixed and often contradictory on the topic of size (excluding all other factors), so I consider the narrow vs. wide debate more about personal preference.