The lightest most minimalist easy to slip on shoe in the world will slow you like a concrete block if it isn’t comfortable.
If you’re in the market for a pair of flats just get the pair that fit the best and feel the best.
Some flats have minimal support features others are socks with a wedge of EVA glues to them…don’t concern yourself too much with the details…just get the ones that feel right.
i use the asic ds trainers as a training and racing shoe. i’ve done 2 ironman’s on them as well. i don’t think they’re a racing shoes though, per se. i think they’re classified as a performace trainer. the ds racer is the racing shoe. hope this helps.