Some background before I get into the juicy parts: I'm a team lead for a scrum team with four (used to be five) devs, two QA, a product owner, and a tech writer. We do two-week sprints, and it's a medium/large company.
I was fortunate enough to work with a very high performing dev on my team for a couple big projects which translated to about a year. He was a big team accelerator. By that I mean he finished his stories and bugs for the sprint before most others and quickly diverted to helping the rest of the team (including QA) with in-sprint stories/bugs/tasks, mentoring less-proficient devs, or working ahead in the backlog if everything in-sprint was well on-schedule.
This led to a very long "green" streak for the team. We hit our mark so consistently that management grew suspicious and started asking why we were so perfect. I explained the working ahead thing combined with the QA bottleneck, and they were satisfied.
Fast-forward to the end of a big project and the beginning of another, and my high performer gets an opportunity to become a team lead for a new feature team being formed. He jumps on it, and we're all very happy for him... until the first two sprints without him. The impact he had on our velocity was much higher than any of us expected and led to a couple failed sprint commitments.
We tried to commit to 20% less, and then 25% less, but his impact on the team was actually closer to 30% (with 5 devs at the time). His influence on our productivity was far beyond simply being one-fifth of the devs. He gave great advice, helped to decompose projects into bite-size pieces, seamlessly put on his QA hat when it was helpful, and so many other hard-to-measure behaviors that added up to a very significant contribution to the team.
We're back in the green now, but we certainly learned our lesson. Everyone has stepped up and is settling into a healthy rhythm. We're learning how to put on our QA hats, break down stories to even smaller pieces, and we're being a little bit more pessimistic in our commitments and estimates.
I'm thrilled for his career progression, and I'm thankful for all the time we got to spend working together, but the hardest part of having an amazing teammate is when they inevitably move on.