In your benchmarks, KDB's max on longs takes twice as long as sum on longs. I am not able to replicate this with the most recent version of KDB (2020.03.30).

    $QHOME/l64/q -s 4
    KDB+ 4.0 2020.03.30 Copyright (C) 1993-2020 Kx Systems
    l64/ 4()core 516718MB <snip>

    q)zz:1000000000?1000j
    q)0.01*system"t do[100;max zz]"
    257.52
    q)0.01*system"t do[100;sum zz]"
    251.95

There's a marginal difference here rather than double.

For the version of KDB with the regression bug:

    q)0.01*system"t do[100;max zz]"
    512.21
    q)0.01*system"t do[100;sum zz]"
    254.24

Which starts to look more like your numbers.