By using C for the hot paths (e.g. HTTP parsing) and python for business logic you can get code that is both faster and easier to maintain than, say, Java.
HTTP is not the bottleneck of anyone's application. Any interesting business logic will almost certainly cause Python to perform worse than Java, Go, etc. I'm also skeptical of the maintainability claim, because I write Python professionally.
Writing web code in c isn't my idea of easy to maintain
If you used this framework you wouldn't be maintaining web code in C.
That's why you write in Python and use this library
The Python part would slow the whole thing down again.
Anything not written in assembly would slow things down. But very few developers write web apps in assembly these days.