You need to post more information. There's no way to tell what's going on from just that little bit of information.
Here's what the documentation on the class mapping says (please go read it):
Quote:
dynamic-update (optional, defaults to false): Specifies that UPDATE SQL should be generated at runtime and contain only those columns whose values have changed.
So what you're saying is that you've marked the class as dynamic-update="true" and that it's not dynamically generating an update statement that only includes the delta changes? If so, check to see if Hibernate is performing a query to determine which fields have actually changed - hibernate has to do that to find out which columns need updating.
Also, turn on Hibernate debugging so that you can see exactly what it's doing. If you're going to use Hibernate effectively you really need to get comfortable with its log output.