Whats is performance? how do we measure it?

To understand performance the is need to define goals and objectives of both employer and employees.
Performance can only be measured as per the agreed standards or goals, responsibilities and duties.
to get the desired performance employees must be motivated to perform by providing job satisfaction and tools of trade.

First you should decide what should be, what should be my cost if I sell x units, what is the relation between cost and revenue, is it variable or fixed cost.
When you have already such information you can start measuring or evaluating the performance.
In CMA study materials you will find too much information about performance analysis.
Regards, Ali

performance it is the comparison between expected date that the management accountant prepared it based on the historical and expected information and actual data this comparison get us the variances in the performance on the company and it is measure the variance if it favorable or unfavorable

it is comparison between expected job outcome and actual job outcome.
how to measure it depend of type of outcome for example a factory expected to produce 100 unit/hour if the factory produce less then the performance is poor if it produce more then the performance is better.
in real life it is much more complicated because there are many outcome to measure.
besides subjectivity always find its way into performance appraisal, only an HR professional can minimize the effect of subjectivity.