Paul Randal shares some thoughts about synchronous stats updates:
The SQL Server query optimizer makes use of statistics during query compilation to help determine the optimal query plan. By default, if the optimizer notices a statistic is out-of-date because of too many changes to a table, it will update the statistic immediately before query compilation can continue (only the statistics it needs, not all the statistics for the table).
Note that “too many” is non-specific because it varies by version and whether trace flag 2371 is enabled – see the AUTO_UPDATE_STATISTICS section of this page for details.
Read on to learn more, including the problems that synchronous stats updates can cause, what you can do to avoid them, and ways you can tell that synchronous stats updates are a problem in your environment.