Unless there's an index on the EncryptionID of both tables, the answer would be "[font="Arial Black"]very [/font]slowly". 😉

I also seriously question setting the last update column to NULL. You are, after all, doing an update... perhaps an "upsert" where you do an update if the EncryptionID is present in both tables and an insert if it's in the source table and not in the target table.

I also wouldn't do it just 5,000 rows at a time. That's 2000 updates and it's just going to be slower than you expect. Under the right conditions (which includes the correct indexing, of course), SQL Server can update a million rows in just several of seconds.

With that in mind, we need to know what the indexes are on these two tables. The CREATE INDEX statements would be the best thing to post so we can tell if they're unique, what the fill factor is, etc, etc. We also need to know if we can build a control table (adding a control column and index to the 10 million row table would be even better) and whether or not this is just an update or if it's actually supposed to be an "upsert".