Session created saved and opened using 'Session' plugin. Session has four files, namely an index.html, a Markdown file, and two plain text files, all of which are very small in size (3.7k, 0.2k, 0.3k, and 3k). The HTML file lives in a local directory which is sync'd to the MEGA service, the Markdown in a local directory sync'd with Dropbox, and the two plain text files in different directories, both sync'd with MEGA.
Typing in any file *except* the HTML is normal performance, but in the HTML file there is a very noticeable lag between my typing and what appears on-screen in the file. I can type, look down at the keyboard, look up again, and the text is still being drawn to the screen, character by character.
I suspected my user.json, so stripped it back to just one line, the path to the Python Library ("pylib__mac"), re-started Cuda, but the problem is still there, so might it be something in the default.json?
With an identical user.json and pretty much the same plugins on Linux, I do not have the lag issue at all, it is as quick as ever.
Interestingly, if I open any *single* HTML file for editing, there is no lag. The issue seems (and I'm guessing here remember) to be when I am using a Session, and one of the files in that Session is an HTML file.
Finally, not sure if it is significant, but Macs are using Python 3.9.6, my Linuc kit 3.7.3.
Suggestions welcomed

Tony