Simply replace the unfilled areas such as "" and "[date]" with your information.
I wish to address some issues with you concerning your performance levels here at [name of organization]. Your personnel performance and productivity rates have shown steady declines in the past [#] [months/years]. Frankly unless improvement is observed, [name of organization] will need to take drastic measures.
These are some of the issues that I am concerned with:
1) [1st performance issue]
2) [2nd performance issue]
3) [3rd performance issue]
You are a valuable member of our team so let's address these issues directly to bring your levels back up to par. Therefore I will be monitoring your performance closely over the next few months to help you reach your maximum potential. After you read this contact me immediately so we can set up a meeting to discuss [this/these] issues in detail.