When comparing Phalcon vs Koa, the Slant community recommends Phalcon for most people. In the question“What are the best backend web frameworks?” Phalcon is ranked 10th while Koa is ranked 33rd. The most important reason people chose Phalcon is:
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.
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 Generator support from ground up
Using generators (a bleeding edge feature, even for Node.js) would clean up your code from the mess caused by all those callbacks; making your code more manageable.
Pro Extremely lightweight
Koa is very lightweight with just 550 lines of code.
Pro async/await keywords are supported and has transcended beyond generator functions
Generators functions are of course a huge plus, but at the time Koa team has transcended generations functions and shifted towards async/await style programming. It has made the Koa best framework available in the market.
Pro Development team has a proven track record
Koa is developed by the team behind a widely used node.js framework (express.js).

Pro Built for ES6
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 Community is relatively small
Con Not compatible with express style middleware
Koa uses generators which are not compatible with any other type of Node.js framework middleware.
Con Has no routes separated by HTTP method or URL pattern
Con Wrong Middle ware, security issue
