Salesforce Exam Data Architect Topic 3 Question 9 Discussion

The data architect for UC has written a SOQL query that will return all records from the Task object that do not have a value in the WhatId field:

Select id, description, Subject from Task where WhatId != NULL

When the data architect usages the query to select values for a process a time out error occurs.

What does the data architect need to change to make this query more performant?