When comparing Phalcon vs FastAPI, the Slant community recommends FastAPI for most people. In the question“What are the best backend web frameworks?” FastAPI is ranked 4th while Phalcon is ranked 10th. The most important reason people chose FastAPI is:
It has a simple but powerful dependency injection system, it can be used to handle authentication, per-user rate limiting, authorization controls (e.g. with roles), etc.
Specs
Ranked in these QuestionsQuestion Ranking
Pros
Pro Extremely fast
Since the framework is and extension built in C it's extremely fast and efficient. It's actually one of the fastest PHP frameworks, and according to some benchmarks it's the fastest framework out there.
Pro Open source
Phalcon is open source and is available under the BSD License.
Pro Uses Volt template engine
The Volt template engine, which is embedded into Phalcon itself takes it's inspiration from the Jinja template engine and as such it's nice to look at, with a clear and understandable syntax.
Volt also compiles very fast, like Phalcon itself, so it avoids being a bottleneck for the framework's overall speed.
Pro Very flexible project structure
You can set up the base project as you want. It's very flexible
Pro Clean and light project code
Since the framework code is not in the project directory, the code is light and clean.

Pro Customizable with Zephir
Zephir is a high-level language designed to create PHP extensions easily by PHP programmers with no knowledge in C.
Zephir does this by compiling directly to C and then the C program is in turn compiled to be run as a PHP extension. This, coupled with the fact that Zephir's syntax is very similar to PHP makes it a perfect way for PHP developers to use it for customizing Phalcon.
Pro Loosely coupled components
Some components can be used as standalone packages like models, views, etc..
Pro Dependency injection
It has a simple but powerful dependency injection system, it can be used to handle authentication, per-user rate limiting, authorization controls (e.g. with roles), etc.
Pro Standards
It is based on standards: OpenAPI, JSON Schema and OAuth 2.0.
Pro Data validation
It validates the data using the types you declared. Even in deeply nested JSON requests.
Pro High-performance
It's based on Starlette and Pydantic, so, it's one of the fastest Python frameworks.
Pro Editor completion
It is based on Python type declarations, so, editors and tools can give great support. Including type checks and autocompletion everywhere.
Pro One of the fastest growing communities
Pro Fast is really fast (!)
It's easy to develop API based applications in Python on deadlines for Android and IOS Development.
Pro Automatic docs
It generates interactive API documentation automatically from your code.
Pro Database independent
It's independent of database or ORM, but compatible with all of them. Including relational databases and NoSQL.
Pro Async IO / optional
It's based on Async IO, which gives it high concurrency. But you can use non-async libraries and it runs them appropriately.
Pro World class documentation
It has some of the best documentation of any framework.
Pro WebSockets
Because it's an async framework, it can handle async-native protocols like WebSockets.
Pro OAuth 2.0
It has integrated support for OAuth 2.0. Including declaring required scopes per endpoint. So, you can easily integrate it with external OAuth 2.0 providers or build your own with it.
Pro Background tasks
Included support for background tasks, thanks to being based on Starlette.
Pro Supports GraphQL
Python's graphene library is included as an optional dependency meaning that GraphQL API's are supported out of the box, with no additional tweaking needed.
Cons
Con Not for shared hosting
Phalcon needs root access to install the PHP extension which is written in C. Developers who plan on using Phalcon must use VPS or Cloud Hosting with root access available.
Con Require good programming skills
Not so easy to use if you want to gain the best from it.
Con Debugging requires knowledge of C
You need to be a C programmer to debug Zephir or C code. Or if Phalcon is not maintained anymore and you have a problem and don't have much skills in C, you will be hard-pressed to find a C programmer to fix it.
Con Smaller community
Since FastAPI is relatively new, its community is smaller than Django Rest Framework. But it can grow with time.
