You might be wondering how, in fact, a bad review could be a blessing. Well, hear me out. Let's face it: absolutely no one looks forward to performance reviews, which typically occur right before the new year. It's even more frightening to hear your boss mid-year say, "We need you to improve your performance," regardless of the industry. So why is this a good thing?
Invested in you. If your boss really didn't care about your value, they would say nothing and start looking for your replacement. Plenty of superiors will keep quiet and let your poor work slide, either saying they are too busy to train you or praying someone else will. An employer's constructive critique of your performance indicates that they are invested in you. If they didn't think you have a future at the firm or actually have the potential to improve, they would keep their mouths shut.
Fear silence. If you can hear a pin drop every time your boss is around you, that's when you should be scared. If your boss is silent towards you but complimentary (or course-correcting) toward the rest of the team, you have a big problem. If your boss is quiet towards everyone, though, then that's okay – if it's her personality. Perk up your ears to hear what she is telling everyone else and how often – daily, weekly, monthly. For the most part, you want to hear praise from your boss every few months. She'll do this to reinforce your behavior. If your performance is slipping, and she sees potential in you, she'll speak up. If your performance is taking a tumble and she's quiet – be worried.
Drowning in work. You might immediately think that you are in the dog house if your boss is dumping on the work all of sudden. It just depends on the type of work. Is it mainly clerical, last-minute and ultimately not desirable? These are all negative signs. If it's working on an important presentation (because someone else was under performing) it can be a good sign. If you seem to always be getting the grunt work while the rest of the team is off to a happy hour around 4:00, you are no longer in your boss's good graces. It would be a good idea to reevaluate your position. Are you happy? Are you doing the best you can do? If you want to stay, seek your boss and ask for constructive criticism. If she's still quiet and your situation hasn't improved, it's time to look for work elsewhere.