No good reason. You've interrupted "what people said is true" with "about how time flies fast." It doesn't seem necessary to actually
set it off, but, it seems like when you read it, and especially when you speak it, you need to indicate where the joint comes. If it were a little shorter, the brain (my brain in particular) could easily grasp it without the comma: " . . . what people say about time is true." Or, " . . . what people say is true."