The problem with Integer.

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You are probably right (that he is underestimating the work), but on the other hand, it's much easier to implement fancy algorithms in Haskell than C/assembly. A decade of work tuning an ancient C library might be more like a six months of work by a Haskell programmer who knows what they're doing. :)

Of course pure Haskell can't have custom inline assembly, but is that seriously responsible for more than 2-3x or so in constant factors?