You can drastically improve the speed of GNU sort by using a few of its command line options.

From the man page:

-S, --buffer-size=SIZE
    use SIZE for main memory buffer
    SIZE may be followed by the following multiplicative suffixes: 
    % 1% of memory, b 1, K 1024 (default), and so on for M, G, T, P

--parallel=N
    change the number of sorts run concurrently to N

-T use DIR for temporaries, not $TMPDIR or /tmp;

You will probably also want to use:

Set LC_ALL=C to get the traditional sort order that uses native byte values.

Example:

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export LC_ALL=C
sort -S 8G --parallel=4 -T /mnt/fast_ssd/tmp unsorted.txt > sorted.txt

Note, this will not use 8 gigs of memory unless it is needed (e.g. the file is very large). After sort consumes the given memory limit, it will then start using tmp files to complete the sort. In this case, it is beneficial to use -T to place sort’s tmp files on a fast disk. Your should also not exceed the number of cores your system has when using –parallel.

References: