What’s one question I should always ask my employees during performance reviews and why?

I always ask: ‘If you can change one thing about the company or your role to improve the company, what would you change and why?’  Each employee has a slightly different perspective on your business and everyone has an idea as to what their largest perceived pain point is.  This is invaluable information that will not improve your company, but will also improve employee moral.  Your employees are your most valuable asset! Invest in them and listen to them!