Kendra Little explains what happens when you update a table’s statistics with FULLSCAN:
On my test instance, the command that uses the default sampling takes 6 seconds to complete.
The command which adds “WITH FULLSCAN” takes just over five minutes to complete.
The reason is that those two little words can add a whole lot of extra IO to the work of updating statistics.
Kendra shows the query plans for each statistics update in some detail. It’s a very interesting post, well worth taking the time to read.