I do not use a graphical editor (called "text editors" in Moodle), so can not help. I would look in three directions:
2. Server performance issues
3. Network issue (configuration or speed)
What happens if they set "Text editor" to "Plain text area" in their profiles?
What does https://docs.moodle.org/27/en/Debugging say?
Word Limit in TinyMCE Editor
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