Video-Streaming-with-Flask | Streaming video with Flask Web Server | Websocket library

I am experimenting with the code found in Miguel's tutorial on streaming jpg images with flask. part1 part2

I tried to add socketIO to the example. but I found that it leads to heavy lagging even if a monkey patch is used. I boiled down the problem to this short example seen below. If I start the socketio instance then I see in the terminal that the generator provides consecutive frames at a regular interval, but in the browser only about every second image shows up. If I use app.run(...) then everything works fine. Using socketio with async_mode="threading" also fixes the streaming.

What am I missing?

templates/index.html