LazyDFU

DataFixerUpper, usually shortened to DFU, handles converting worlds from older Minecraft versions to newer ones. It starts before the game window appears, and during that setup it builds conversion rules for every version between the old and the launched one. That work runs in the background but is heavy on the CPU and memory. LazyDFU lets DFU start, but stops it from caching all of those rules up front, so they are compiled only when a world conversion requires them.

Because the rules are built on demand, a conversion may pause briefly the first time it runs, while the migration rules are compiled. Afterwards there is no ongoing cost. Open source under the MIT License, the mod works on the client and on the server, so either side can use it. Builds cover 1.14 – 1.20.6, and version 0.1.3 adds a client-side step that stops the client from initializing every rule.