Personally, I don't like the pentagon. It is too general and has no real connection to SFML. Also, there are thousands of logos like that, a good logo should be unique in a way. And it should also be recognizable without the text.
Recently, I have spent some thoughts about a logo that could be associated with SFML's philosophy "simple and fast", and I came across an object that clearly fulfills these properties:
An arrow. I experimented a little bit with a logo that incorporates an arrow. I'm not a great artist, but tell me whether you like the general idea. The following pictures are scaled to half size, in order to be displayed without scrolling.
Raw:
Contour and shadows:
Shiny:
Only arrow:
Arrow with more shadows:
3D model of the arrow: