I'm not sure if this was the first, but a first big one. I added DOM reorder support for Riot 2.0 (it lacked it completely): https://github.com/riot/riot/pull/384
Later we made it better: https://github.com/riot/riot/issues/484
..and better: https://github.com/riot/riot/issues/990
..but reordering was still a bit slow. Then I realized the reordering itself wasn't the cause of slowdown, but something else buried in Riot (hopefully fixed). To prove that, I wrote my own view library FRZR, which became RE:DOM.
RE:DOM is currently one of the most smallest, simplest and performant view libraries out there. I've learned a lot working with the awesome Riot team!