About a month ago, I merged a complete refactoring of the sub-requests management of Symfony. In fact, I created a whole new sub-framework to handle the rendering of resource fragments via different strategies. [...] Besides classical master requests, the HttpKernel component is now able to handle sub-requests. Let's me sum up the different strategies that are available: internal sub-requests, ESIs, HIncludes, and SSIs (in 2.3).

He talks some about each strategy and what situations they're best suited for and some sample code that shows the use of the "standalone" and "strategy" parameters. There's also new methods you can call (like "render_esi" or "render_hinclude") to work with the new features as well.

One of the big advantages of both Symfony and Silex is that they are based on the Symfony HttpKernel component. [...] But handling HTTP requests with the Symfony HttpKernel also gives you a lot of free and optional features like a built-in reverse proxy written in PHP, easy handling of ESI and Hincludes (via the Fragment sub-framework), or the gorgeous Symfony web profiler.

The profiler provides information on which components were accessed, their memory consumption and how long they took to execute. The integration happens via the WebProfiler provider for Silex and is already integrated into the Silex skeleton base application.