Just speaking for myself, but I think better when I'm not pinched between two projects both demanding preference and priority over the other. I can maintain a clearer mind when I don't have people breathing down my neck about deadlines they set and then broke.
Sure, one learns to navigate those situations and maintain an even keel, but still one is obligated to respond in a professional manner to those inquiries and meetings and other emotionally manipulative games people play to get their way when in a crunch. Working through those things still requires time out of day and energy out of mind. Both of those latter resources are limited.