I'm excited to see the Python web community pick up Cython more and more to speed up their web frameworks.
uvloop as a fast drop-in replacement for asyncio has been around for a while now, and it's mostly written in Cython as a wrapper around libuv. The Falcon web framework optionally compiles itself with Cython, while keeping up support for PyPy as a plain Python package. New projects like Vibora show that it pays off to design a framework for both (Flask-like) simplicity and (native) speed from the ground up to leverage Cython for the critical parts. Quote of the day:
"300.000 req/sec is a number comparable to Go's built-in web server (I'm saying this based on a rough test I made some years ago). Given that Go is designed to do exactly that, this is really impressive. My kudos to your choice to use Cython." – Reddit user 'beertown'.
Alex Orlov gave a talk at the PyCon-US in 2017 about using Cython for more efficient code, in which he mentioned the possibility to speed up the Django URL dispatcher by 3x, simply by compiling the module as it is.
Especially in async frameworks, minimising the time spent in processing (i.e. outside of the I/O-Loop) is critical for the overall responsiveness and performance. Anton Caceres and I presented fast async code with Cython at EuroPython 2016, showing how to speed up async coroutines by compiling and optimising them.
In order to minimise the processing time on the server, many template engines use native accelerators in one way or another, and writing those in Cython (instead of C/C++) is a huge boost in terms of maintenance (and probably also speed). But several engines also generate Python code from a templating language, and those templates tend to be way more static than not (they are rarely runtime generated themselves). Therefore, compiling the generated template code, or even better, directly targeting Cython with the code generation instead of just plain Python has the potential to speed up the template processing a lot. For example, Cython has very fast support for PEP-498 f-strings and even transforms some '%'-formatting patterns into them to speed them up (also in code that requires backwards compatibility with older Python versions). That can easily make a difference, but also the faster function and method calls or looping code that it generates.
I'm sure there's way more to come and I'm happily looking forward to all those cool developments in the web area that we are only just starting to see appear.
Update 2018-07-15: Nexedi posted an article about their attempts to build a fast web server using Cython, both for the framework layer and the processing at the application layer. Worth keeping an eye on.