Re assign Records Issue in User entity

Hi All,
We have an issue in reassigning the records to other user in any user record.

Steps:

1. Login to CRM, Open any User record

2. We have a system button called "Re-assign Records" in the ribbon

3. I clicked on the button and re assigned the records from User A to User B

4. Its taking huge time to re-assign the records to user B which is more than an hour and the reassigning popup is not ending

Tried with test user account with less records to re assign but still the same issue.

Can anyone please help me on this?

Thanks