Just to add... If everyone in the office and even outside clients are not only noticing the slowdown but also able to point at the root cause (your coworker),

Just to add... If everyone in the office and even outside clients are not only noticing the slowdown but also able to point at the root cause (your coworker), you really don't need to stick your own neck out there to let them know something they are aware of. It's already known. Let the situation come to fruition at it's own pace.