People say that ImageMagick is much faster. At best just compare both libraries and measure that.
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Prepare 1000 typical images.
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Write two scripts -- one for GD, one for ImageMagick.
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Run both of them a few times.
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Compare results (total execution time, CPU and I/O usage, result image quality).
Something which the best everyone else, could not be the best for you.
Also, in my opinion, ImageMagick has much better API interface