GNU sort uses the following rule to determine the size of the memory buffer used for the mergesort algorithm:

buffer_size = min(1/8 * physical_RAM, free_memory)
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That's somewhat conservative, specially if the machine you are using is not very loaded. Increasing that buffer size will make the sorting much faster. For instance

sort -S 3.5G ...
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Another way to increase the speed of the operation, is to reduce the size of the file using a more compact encoding. For instance, representing numbers in binary format instead of as ASCII strings will reduce its size to 1/5; DNA sequences can be reduced to 1/4; enumerations to 1 or 2 bytes, etc.

This kind of compacting will introduce "\n" characters in the stream that need to be escaped. A simple way is to perform the following expansion:

my %expand = ( "\x10" => "\x11\x11",
             "\x11" => "\x11\x12");
s/([\x10\x11])/$expand{$1}/g;
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