yes, we expect professional software developers to have basic maths skills

"what is a prime number" is taught to 7 year olds, it's not vector calculus

what else would you consider to be an unreasonable thing for an employer to require?

reading and writing skills of a typical 7 year old?

+1 is not a good idea since ~half of all numbers are effectively non-prime simply by being even numbers.

You can double the speed by using +2 without using any fancy tricks, just changing a single character.

Common approach is to use square roots, this reduces the runtime. Recommend checking out project euler if you like solving hard math-code-o(n)-puzzles.