On the top of what has been already mentioned I would like to recommend you for practicing and improving your mental math you can find also some apps and tools, besides Victor Cheng’s math tool (Magoosh's mental maths app, Mental math cards challenge app etc).
The next important thing is that rather make it on the paper to structure the notes and then to communicate the results clearly rather than make your calculations fast
The time pressure in case interviews is severe and you cannot afford to waste time. But to make your calculations right you shouldn’t be in a rush. So, I would recommend you here to work on both - practice with time limitations and learn how to keep your mind peaceful and concentrated (it might be not so popular advice here, but mediations really make their job here ).
Be comfortable and confident to state your answers not as a questions. Interviewers notice this, and this will not give a credit.
If you need any further help or career advice, feel free to reach me out.