I have 6 text files of the format SHA-1<tab>7_hex_chars<newline> and each file has 9,000,000 lines. The 6 files are in the same order, with the SHA-1 being the key column of all the files.

If that's accurate (don't need to sort or worry about unmatched keys), then something like this would be the first thing I would try -- the unix paste and cut commands:

paste file1 file2 file3 file4 file5 file6 | cut -f2,4,6,8,10,12 > hex.
+cols
[download]

That should be pretty close to optimal in terms of performance.