Unless the previous employer is known to fire underperforming employees quickly, and willing to explicitly say so during reference checks, how does a prospective employer know whether a candidate's $n years in $company is a productive one or not?

So you haven't seen underperforming people survive at large famous companies for multiple years? Are you sure you have that much experience in $bigco? How old are you really, since you say the GP is "young"? I wouldn't be so patronizing to others if I were you.