Performance issues of any kind can come at a high cost and can affect almost every metric — from task abandonment to lower adoption and decreased usage. People are less likely to convert, and even if they do they're unlikely to be satisfied with the experience. Of course, making your app or site fast is no small task, and often technical limitations cannot be overcome at a reasonable cost. There are ways to improve perceived performance — but we should always strive for improving real performance as much as possible. Remember — shaving seconds at scale saves lives :)
One of the most effective ways to improve the ux in your digital product is to make it faster, or at least make it feel faster. There is plenty of data showing the huge cost performance issues can incur; such as 88% of people less likely to return to a site after a bad experience. More importantly, by not working to improve the performance of products (digital or otherwise — looking at you CA DMV), we are stealing the most precious asset any of us has — time.