WIP limits are a tried and true method for surfacing bottlenecks. While the concept originated with Kanban, it works regardless of your process. In Kanban, WIP limits are typically assigned to each work phase (e.g. To Do, Doing, Done). But the same idea can be applied to people.
Capping the amount of work in progress is sensible for a number of reasons. WIP makes finding bottlenecks easier—in this case, who may be overloaded—and it affirms the simple fact that trying to do too much at once leads to less actually getting done, and more team frustration.