why are some horses better doers than others?

I may have read these posts totally wrong but the good doer horses have a poor metabolism not good- the faster metabolism the more calories are burnt.
However pain worry stress also have a lot do with it that's why horses like this tend to be poor doers (fast metabolism) as they burn calories just thinking- same for some people i for one know if Im stressed i can eat what i want as i loose weight as have a fast metabolism.
 
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