From my experience of professional mathematicians at university, arithmetic was usually their worst skill. They'd quite often struggle (for a short moment) to multiply, say, 12 and 16 together. The public perception of a mathematician is somebody who remembers Pi to a 1,000 digits. A real mathematician doesn't care less.
This. So much this. Arithmetic is bookkeeping, it's turning an elegant abstract process into an ugly solved instance through rote mechanical labor.
In most of my college math classes, I largely 'got' it, but what killed me, and many other students, was the arithmetic. We could take multiple integrals, but dammit, 2*3 /= 5. Thank God for partial credit.
