When editing a document that becomes too large, you can split it into sub-notes:

1. Select the desired text and cut it to the clipboard.
2. Create a new sub-note and name it.
3. Paste the content from the clipboard into the sub-note.

Trilium can automate this process. Select some text within the note, and in the selection toolbar, click the scissors icon for the "cut & pasted selection to sub-note" action. The heading is automatically detected and the new sub-note is named accordingly. You can also assign a keyboard shortcut for this action. This functionality is available through the block toolbar icon.