How slow is "slow"?
The main problem with this is that it would create an enormous entry in the database's log file (in case there's a power failure half-way through the update, the database needs to log each action so that it can rollback in the event of failure). This is most likely where the "slowness" is coming from, more than anything else (though obviously with such a large number of rows, there are other ways to make the thing inefficient [e.g. doing one DB roundtrip per update would be unbearably slow], I'm just saying once you eliminate the obvious things, you'll still find it's pretty slow).
There's a few ways you can do it more efficiently. One would be to do the update in chunks, 1,000 rows at a time, say. That way, the database writes lots of small log entries, rather than one really huge one.
Another way would be to turn off - or turn "down" - the database's logging for the duration of the update. In SQL Server, for example, you can set the Recovery Model to "simple" or "bulk update" which would speed it up considerably (with the caveat that you are more at risk if there's a power failure or something during the update).
Edit Just to expand a little more, probably the most efficient way to actually execute the queries in the first place would be to do a BULK INSERT of all the new rows into a temporary table, and then do a single UPDATE
of the existing table from that (or to do the UPDATE
in chunks of 1,000 as I said above). Most of my answer was addressing the problem once you've implemented it like that: you'll still find it's pretty slow...