When a single user owns more than 10,000 records, it is called 'ownership data skew'. It can cause performance issues if the user is moved around the hierarchy, or if they are moved into or out of a role or group that is the source group for a sharing rule. The following actions can be taken to prevent long-running recalculation of access rights:1) The ownership of records can be distributed across a greater number of users to decrease the chance of long-running updates.2) If possible, the user should not be assigned to a role in the role hierarchy.3) The user should be placed in a separate role at the top of the hierarchy.4) The user should not be moved out of the top-level role.5) The user should be kept out of public groups that could be used as the source for sharing rules.

https://resources.docs.salesforce.com/sfdc/pdf/draes.pdf