The bug

World conversion even from 1.12.2 is very slow.

For example the attached world which contains only a freshly generated, immediate spawn area across two region files and takes around 24 seconds to load from an SSD and overclocked (4.5 Ghz) desktop core i7 processor.

It is likely that the vast majority of Minecraft players, and indeed many server owners are running on hardware far less powerful than this, which would lead to a poor gameplay experience.