If the role often requires impromptu whiteboarding with absolutely no context before the meeting, include that in your interview loop.
If it doesn't, cut this garbage out of your interview loop.
Impromptu whiteboarding exercises bias toward those who excel at leading impromptu ideation and ad-hoc problem solving on a topic provided by others. In my experience, this is a minority of the people you'll hire. The majority benefit from preparation in advance and additional time to process their thinking.
A better way: Give candidates context for the scenario much the same you would if they were already an employee. Encourage them to utilize tools — yes, whiteboards too if appropriate — that they've found successful for collaborative meetings.
You might even try turning the tables: Let candidates decide the topic and the rest of the room (the interviewers) participate impromptu.
On whiteboarding exercises in interviews…