I finished a Lua game engine with SFML.
just need to the port to android (almost done) and other platforms outside windows and finish the docs.
My Lua game engine run very very fast with SFML. You can create without problems the most demanding 2D games with it.
SFML have a very good performance, and an excelent API, without saying that we have here sfe::Movie, LTBL2 , Thor, SFGUI, TGUI, Selba Ward, ETC.
I probably made a GUI for my engine with TGUI or SFGUI.
SFML is full of tools and have clear docs, SDL not to much, the only decent tutorials for advanced topics are from lazy foo, and are ugly in comparison with SFML.
The performance of graphics libraries based on OpenGL is almost the same. The unique thing that will really boost your engine is use raw openGL for specific things. And only with Vulkan you'll have portable graphics with possible multi-thread rendering.