If there is no index on the URL column on the table you are pulling the data from then you will probably end up with a row by row table scan. Very slow.

You could create a temporary index and drop it when the updates are complete. That is assuming this is a one time update. If it will be run several times, it may not hurt to leave the index in place.

Take a look at the execution plan and it will tell you what is taking the most time and give some recommendations as to how to speed things up.