Investigate freeze/slowness on notebook cell execution

[DonJayamanne]

Notes

1. Large notebooks with Mardown cells seems to somehow slow things down..
   Jupyter for vscode continues to be slow (for large notebooks with mardown cells & large outputs) vscode-jupyter#14459
2. Large notebooks with a large number of cells and/or large outputs seems to hang VS Code as well
   Jupyter for vscode continues to be slow (for large notebooks with mardown cells & large outputs) vscode-jupyter#14459 (comment)
   1.87 still very slow for big jupyter notebook vscode-jupyter#15352 (comment)
3. Saving is slow
   VSCode jupyter extension (on Insider) appears to take a long time to get saved after being used for a while. How to mitigate? vscode-jupyter#15366
4. Possible rendering of outupts even when they are not visible
   Possible perf issue due to unnecessary rendering of outputs when outputs are not visible #207748
5. When hitting Run All Extension host does not get message from VS Code for a few seconds, hence perceived delays in execution
   Notebook execution seems slow as clicking Run All does not reach extension host for a few seconds #207755
6. Completions are slow, and executions are also slow
   Slow edits and completions when editing Python code #210528