We the members of this forum are outsiders. We do not know the background of the case so that we can provide the draft of the mail.
What is the nature of your industry? What is the nature of the work that employees do? On what parameters the performance will be evaluated in the performance matrix? Was the evaluation regular practice or something new? Have you added few new parameters in the matrix?
The most important question is why you would like to measure performance every week or fortnight? Is measurement at that frequency a regular feature or this is new introduction? Have you created supporting mechanism (or do you have software to measure) the performance at that frequency and that too with 100% accuracy?
There are so many questions associated with your post. Providing the draft is easy but the circulation of the mail or circular should not generate further questions from the employees. Hence the above questions.
1st October 2018 From India, Bangalore