Streamlit slows down with many nested blocks

[raethlein]

Checklist

• I have searched the existing issues for similar issues.
• I added a very descriptive title to this issue.
• I have provided sufficient information below to help reproduce this issue.

Summary

In this forum discussion, we investigated an issue where a user app became slower after release 1.35.

It looks like the CSS styling introduced in this PR 5 is problematic and started to make rendering bigger apps using many blocks way slower.

Reproducible Code Example

No response

Steps To Reproduce

The issue can be reproduced by running the user's app from this repo: https://github.com/diogovalentte/mantium/tree/main
via Docker locally. The user has provided a database than can be copied to the postgres container in order to populate the app with enough data. I assume you could reproduce it with a simpler Streamlit app, but I haven't found the time yet.
However, when running the app with Streamlit 1.34 and with 1.38 you can notice a significant scroll speed difference even up to a point in which the app freezes. If you comment out the CSS selector from the linked lines of code above, the issue seems to be fixed.
Here are a few example commands (update the paths) where you can easily make the changes locally on your Mac and copy them into the container to test changes:

# [On your local Mac] 
## Build the frontend
cd frontend
yarn build

## copy the build artifacts into the container
docker cp ~/Projects/streamlit/frontend/app/build/ mantium-dashboard:/usr/local/lib/python3.10/site-packages/streamlit/static/

## Open bash in container
docker exec -it mantium-dashboard /bin/bash  

# [In container]

## Remove the current frontend assets
rm -rf /usr/local/lib/python3.10/site-packages/streamlit/static/*

## Move the copied-from-Mac frontend assets into the container
mv /usr/local/lib/python3.10/site-packages/streamlit/static/build/ /usr/local/lib/python3.10/site-packages/streamlit/static

Expected Behavior

Scrolling through the app should not make the browser window freeze and/or be super slow.

Current Behavior

No response

Is this a regression?

• Yes, this used to work in a previous version.

Debug info

• Streamlit version:
• Python version:
• Operating System:
• Browser:

Additional Information

No response

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