There are a lot of great comments in here already but I'll add my 2c anyhow because it's something I have been thinking a lot about lately. Python is my absolute favorite language without a doubt. But Python is dying for ME I think there is a lot of value in having front and back end code in the same language. Python is significantly slower than JavaScript and I just don't want to have to run 2x the number of servers…
I don't think there's anything inherent about either language that makes it faster or slower than the other, and it's not really fair to compare one high-performance implementation (Node) with another specific implementation (CPython) when there are other options. For example, have you compared Node vs. PyPy? In my experience they're very close.