I know very successful quant traders who are so poor at mental arithmetic that they reflexively use ipython to compute 20% of $100M.

Writing correct trading algorithms has more to do with knowing what your blind spots are than being good at math party tricks.

It depends on the type of trader. Before the days of automation, it was essential. If you could do the mental math the fastest, you would get the best trades. Not surprisingly, this skill matters less when all your trading activity is conducted by algorithms.