Thank you all for thoughts and suggestions. I've been tracking them as
they appeared, but being busy with university assignments, couldn't
try them out yet.

In the meantime, however, Karol Samborski investigated the issue
further and found the cause of poor performance -- somehow using
HashTable.newHint doesn't play well with HashTable.update. Simply
changing newHint to hint gives me execution time of about 12s (on my
rather slowish AMD Fusion 1.6GHz laptop) with a dataset of ~1.35mln
words.

It's evidently a bug, as newHint would hardly be expected to slow
things down. I'm yet to discover how to properly report it.

Thanks again to all who responded and especially to Karol for solving
the problem.

Best regards,

Radek Szymczyszyn

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