I would recommend Haskell for speed of development and correctness of implementation, but (probably) C for speed. You can of course combine the two with the FFI, but I don't know how trivial it is to pass Haskell graph structures to C and back.
If you use a C library for speed, you want to design the junction so that you make relatively few calls across the border and pass only a small amount of data across the border. For example, if manipulating bitmaps, you would try to keep the bitmap object in the C world rather than copying it back and forth between C at every step. A nice property of this kind of interface is that you wouldn't have a Haskell graph structure to pass back and forth - the graph would live entirely in C. The only real complication you'll run into is that you'll probably need to use ForeignObj(ects) so that the graph objects can be released when the Haskell garbage collector finds it can't access them from the Haskell heap. Not really hard to do - just need to be careful. -- Alastair Reid