OK, if there are no triggers no the tables, we can exclude problems with query plans as such, but problem is related to the mirroring setup.

One way to resolve it is to give up on synchronous mirroring and use asynchronous instead. But that requires that you fully understand the consequences for disaster recovery - asynchronous mirroring means that there is a risk for data loss in case of a failover.

Another angle is to look at the context of these INSERT statements. I guess since they are problematic, that you are not submitting these in singletons a few times a day, but maybe you are inserting many rows at a time? In such, there is plenty of oppurtunities for improvement!