When something is so bad, uninteresting, or dull that our brains turn to mush, you could describe it as mind-numbingly bad (or dull, or uninteresting). Take your pick!
Words that end in -ly generally refer to the way in which something happens. Mind-numbingly refers to something that happens in a way that makes your brain feel like it's stopped working. An enjoyably long wait might give you time to yourself, or let you bask in nature or relax with a hot drink. A mind-numbingly long wait might leave you bored, frustrated, and exhausted.