This is one of the many nonsense-myths flying around! MBB interviewers NEVER expect mental math! The only thing expected is that you can perform calculations in a clear and correct way, while taking along (!!!) the interviewer! This means that, apart from super easy calculations, you should always do rock-solid written calculations! NOBODY is interested in your mental math skills! It is NOT a relevant category in assessing a candidate's performance, and every MBB interviewer is explicitly trained on that. The interviewer will assess you on whether you are accurate and clear!
In the extreme, very fast mental calculations are even very detrimental to a candidate's performance. Here's why: The interviewer will always assume that the way you behave in an interview is indicative of how you would behave with a client. So if you do all sorts of complicated math stunts in your head - guess what? You become extremely difficult to follow for the client! And at that point it becomes completely irrelevant whether your result is correct or not. If it is hard to follow your analysis, then your performance is poor. End of story.