First question : I know this question seems to be simple, I was wondering when i should go for average or sum, when comparing performance of Measure or dimension?
Second question : In tableau is it better to calculate average inside the calculated field like : sum(sales)/count D(sales)( Aggregated calculations)
or use right click to change from sum to average (Non - aggregated calculations)
the 1st question will be depending on your situation, and some times use avg, some time use sum, and some times use both.
your 2nd question, still depends on your case, but normally is non aggregated calculation as the amount of sales may be the same for different orders.