I originally went with Milkdown (Prosemirror-based) for Atomic, the knowledge base project that I built Atomic Editor for. ProseMirror doesn't provide virtualization out of the box. For shorter notes and even moderately long content it's fine - but atomic supports syncing content from a diverse set of sources and I noticed that long documents were causing delays on initial page load and some lag during edits. I didn't find anything like it with native virtualization that felt right to me so I built Atomic Editor.