A question came up recently wondering why Slim Framework was around 5 times slower than plain PHP.
All frameworks are by definition slower than no-code as there's more going on.
i.e. an index.php of:
is going to be much faster than a Slim application with this code:
This is not an apples-to-apples comparison of course as the Slim application is doing a lot more than the plain PHP one. It supports routing so that it can respond to different URLs (even ones with dynamic parameters in that URL) and HTTP methods, along with the ability to handle errors. It also a set of separate classes so that you can replace specific parts of our code with your own if you want it to do something different while still using the rest of our code.
To actually compare the performance of Slim to plain PHP, you need to make the plain PHP version have the same functionality as the Slim version and you'll discover that the performance difference is much smaller. Of course, custom written, application specific code will always be faster than generic code though. Slim is a micro framework, so already is pretty tight (more to do though and something I'm looking at for 4.0), but by definition, it supports more routing features than you may need.
However, you have to write it all that code yourself and, to be honest, parsing requests, sending responses, dealing with URL routing, formatting errors based on the Accept header, and so on are solved problems that don't add any value to your app if you write them yourself. You're much better off concentrating your efforts on the parts of your app that are unique to your app as that's where it brings value.
The definitive resource on this is Paul Jones' articles. Start with A Siege On Benchmarks. This was back in 2009 and somehow he managed to not test Slim! However, even if you take the fastest framework he measured (ZF 0.2!), you can see that at 180.56 requests per second, that's around 4.5 times slower than the baseline PHP of 829.82 requests/second.
Finally, the speed of this code will pale in comparison to the rest of your application's code, especially if you access a database or a web s service!
I'm interested in making Slim the fastest it can be, but the benefits of having a reliable framework that provides good routing middleware functionality out of the box cannot be overstated.