If you notice that the app is running slowly, your pen response time is diminished, or the app is closing while you are working on a particular note, this may be related to overly data heavy notes, or very long columns of text.
Your device will limit the amount of RAM it makes available for non-OS operations. Text, text size, colors, math, diagram, and sketch objects, along with imported images, etc., are all considered data by your device. So while it is possible to create very long notes in Nebo, it is important to remember that your device will also need to be able to process both the app and the note that you are working on, along with any apps running in the background. See here for more information about RAM: Random Access Memory (RAM)
Try dividing the page you are working on into shorter pages to see if this improves the lag. You can do this by using the duplicate feature to duplicate the page. You can then delete text from each page to match up the content across the duplicated pages. See here for details: Duplicate a page