Why does the delta pass sometimes take a long time?

The Delta pass performs summary date retrieval on all items, in order to determine which items have previously been migrated (using watermarks) and which items still need to be migrated during the Delta pass.

Therefore, even if 95% of the data has been migrated during earlier passes, the Delta pass still requires the full summary date retrieval process to be run, prior to migrating the remainder of the items. Yes, the time for completion will be much shorter than the previous pass but, due to the summary date retrieval process, it may take longer than anticipated if your calculations are based purely on the number of items to be migrated.

In addition, if a mailbox has folders with very large number of items, for example over 10,000 items, this will increase the time spent on summary date retrieval and hence increase the time for Delta pass completion.

MigrationWiz may record very slow migration speeds during a Delta pass, because the speed will be recorded as the amount of data that is migrated over the entire time of migration, which, again, includes the time spent on summary date retrieval.

Example scenario:

A mailbox has one million items.
There are only five new items since the last migration pass, totaling 1MB of data.
An Advanced Option date filter has been configured to migrate data for only the last 3 days.
Recorded speed:

This would be equal to the amount of data migrated over the time that is taken for summary date retrieval, plus the amount of time taken to migrate the five new items. If the summary date retrieval took 59 minutes and the migration of the five items (1MB data) took one minute, then the recorded speed would be 1MB/hour.

For this reason, Delta passes with very few migrated items often record very low performance numbers, such as 0.25MB/hour.