When you're looking for a new job, you'll be reaching out to your network, former colleagues, employers, and recruiters. You will be juggling a lot of balls in the air at once.
This requires a new level of attention and focus that you haven't had to execute for some time.
You'll be performing at your current job while devoting additional time to your future job. Both will be demanding and require significant time out of your already cram-packed day.