First of all, faster isn't always better. One of the engineers I've worked with finished his stories extremely fast, way faster than the rest of the team. But he'd often use quick and dirty shortcuts, and the testers explicitly gave any user stories from this engineer extra attention. He produced different bugs than the rest of the engineers, more difficult to find, but often with a lot of impact.
If there's enough trust in the team, discuss the differences in speed with the whole team and ask them what factors they see that can explain the differences. Some possibilities include personal factors (motivation, drive, perfectionism), skills and knowledge, amount of distraction (soloists vs the people who are always helping others instead of doing their own tasks), or different views on quality/scope/definition of done.
Try to pair the faster engineers with the slower ones (and in general, encourage cooperation) -- both of them can learn from working together.