Why do #designSystem teams take longer to build features than product teams?

If you ask Nathan Curtis, it is because of higher expectations and that is a good thing.

Higher quality expectations:
Design systems must be made to a level of quality to meet or exceed what every adopting team requires. Team(s) resist using features from somewhere else if they feel they do it better themselves.

Read more about it: https://medium.com/eightshapes-llc/design-system-ui-is-more-expensive-than-a-product-teams-ui-f3c3e48c555