According to the official Salesforce guide1, assigning leads to a single dummy user can cause performance issues and data skew, especially if the dummy user owns more than 10,000 records. Data skew occurs when a single user or a small number of users own a disproportionately large number of records, which can affect query performance and sharing calculations. Option B is the correct answer because it suggests creating multiple dummy users and assigning leads to them, which can distribute the load and reduce data skew. Option A is incorrect because assigning the dummy user to the last role in the role hierarchy does not affect the performance or data skew issues. Option C is incorrect because assigning the dummy user to the highest role in the role hierarchy can worsen the performance and data skew issues, as it will grant access to more users and records. Option D is incorrect because periodically deleting leads can cause data loss and does not address the root cause of the problem.