When comparing Slim vs Lumen, the Slant community recommends Lumen for most people. In the question“What are the best web frameworks to create a web REST API?” Lumen is ranked 10th while Slim is ranked 15th. The most important reason people chose Lumen is:
Since it's basically just a minimal version of Laravel, it can be upgraded to a full Laravel app if the need arises. No need for code changes, just import the code to a new Laravel install.
Specs
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Pros
Pro Well organized and thorough documentation
Slim's documentation is well organized and detailed, every concept is thoroughly explained and it is very helpful for both advanced users and beginners.
Pro A good starting point
Slim is minimal and that is a good thing if you want to start from there. It can be easily extended and even supports popular packages that are used in Laravel (like Illuminate\Database (eloquent)) for example.
Pro REST based
REST fans will love the REST based architecture.
Pro Supports tie-ins for Rack-like middleware
Rack is an interface used in Ruby frameworks used to group and order modules, which most of the time are Ruby classes, and specify between them.
Slim uses a simple concept for it's middleware. By wrapping HTTP requests and responses it unifies the middleware into a single method call.
Pro Useful classes
Contains classes for managing requests, responses, cookies, logging, views, HTTP caching, and more.
Pro Flexible
Slim doesn't demand that you stick to a fixed folder structure. As long as you load Slim the right way you can do anything from there the way you like it.
Pro Extremely lightweight
Paired with swoole it's a micro service powerhouse.
Pro Open source
The Slim Framework is open source and is released under the MIT public license
Pro Extremely customizable
You can add any dependency, package or class that you want to use as a contained dependency.
Pro Supports Php 5.3 and PHP 7
Pro Makes it easy to understand the way some abstract functions and classes are built
In Django most things are abstracted, you just call some function or class without knowing how they were built, but with Slim, you end up understanding the way some abstract functions and classes are built.
Pro Hooks for executing code at different points in its life-cycle
Slim supports code hooks for executing functions at different points in time during the application's lifecycle.
Pro Easily upgradable to Laravel
Since it's basically just a minimal version of Laravel, it can be upgraded to a full Laravel app if the need arises. No need for code changes, just import the code to a new Laravel install.
Pro Feature rich

Pro Highest performing PHP micro-Framework
Lumen is benchmarked at 100/rps (Requests Per Second) faster than Slim v3, which used to be considered the fastest and most performant micro-Framework to date with the ability to handle 1800/rps. (1900/rps vs 1800/rps respectively).
Pro Easy to use
Cons
Con Very little consistency among different versions
There have been quite some changes that break the compatibility between Slim 2 and Slim 3. Even if you learned how to work with the Slim 2, you will find that Slim 3 requires re-training.
Con Dependency injection is too weak
It is not really dependency injection, but just a configurable container.
Con Needs strong bases to create dependencies
The dependency container schema of Slim is one of the biggest PROS and CONS of the framework. It is true that this schema brings so much flexibility to add anything, but another thing that is true is that you need to have strong bases of patterns, and an extensive knowledge of your libraries to convert it into a Slim dependency.
Con Too minimal
While it's true that Slim is a microframework, it's still too minimal. When used for throwaway projects or simple prototypes, it's perfect. But in the long run, it becomes less and less useful and you end up in implementing a full custom framework in trying to tackle all the missing features.
Con Built for smaller tasks like APIs
Con It's based off Laravel and inherits its shortcomings
Lumen inherits many shortcomings of Laravel, such as static proxy classes.
Con Made to work alongside Laravel
Lumen as a framework is at it's full potential when used alongside it's older brother. Lumen was created to be used for microservices alongside Laravel, which is used for more user-facing applications. If a project is already using another framework other than Laravel, it would be better to use another microframework for microservices instead of Lumen.
