I found you can actually get pretty good performance out of ImageMagick if you recompile it to use an internal 8-bit per channel colour (QuantumDepth, giving a

I found you can actually get pretty good performance out of ImageMagick if you recompile it to use an internal 8-bit per channel colour (QuantumDepth, giving a per-pixel 'Quantum Size' of 32-bits) setting which is good enough for web graphics. By default ImageMagick seems to have something large like a 32-bit quantum depth (which is equivalent to a fairly massive 128-bit per pixel). I don't know how this compares to GraphicsMagick out of the box but it might be interesting.