Mate, someone needs to tell your company that this is the 21st century.
When someone or a group is not performing or reaching their set KPI's, the last thing you should do is add more pressure by threatening their job. You evaluate the situation together and try to find causes for the issues, while taking circumstances into account.
Based on the above, you either decide the person was provided with all the means necessary and should've have achieved the goals, which is where you generally part ways, or there were circumstances and failures at different levels which explain the failure.
This is where you make a plan and provide the necessary tools and support to mitigate circumstances, correct those failures, and allow that person/group to perform at the level you want.
Going to someone and tell him: "make it better or else...", is akin to setting him up for failure.