Hi
I do not use a graphical editor (called "text editors" in Moodle), so can not help. I would look in three directions:
1. JavaScript incompatibilities in the browser
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?
Text editors
Word Limit in TinyMCE Editor
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