I believe you don't understand very well what McKinsey is testing.
Nobody is interested in your ability to do lightning fast math! The truth is, you don't get bonus points for being very fast - quite at the contrary! The interviewer will assume that the way you behave in an interview is also how you behave vis a vis a client. And the single most important thing when discussing with a client is that you can be followed easily! If you are just doing all sorts of complicated math operations in your head (or secretly use a calculator) - guess what the result will be? Your counterpart will have difficulties following your steps. Hence, this will make your performance WORSE! So what is FAR more important than speed is the rigour and discipline to walk the interviewer through your calculation in a succinct, sharp and cystal clear way! I will always recommend to do calculations in written form and neatly copy the results into a seperate area on the sheet (or another sheet). This is 100x more effective than trying to become super fast in mental math (or "cheat" with a calculator)!
Please be aware: interviews at McKinsey are not an exam, where you have to get to a specific result and this will then determine your grade! It is just about two things: (1) Do you show the right way of thinking, and (2) Are you able to articulate this thinking in a crystal clear way? Using a calculator will have ZERO impact on this.