these are against the latest 4.0 KDB+, after 26 March. KDB before that could not aggregate in parallel implicitly.

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.

sorry, i think we tested with this one:

    KDB+ 4.0 2020.03.17 Copyright (C) 1993-2020 Kx Systems
    l64/ 12(16)core 63960MB
    EXPIRE 2021.03.26

Even without that 2x, all benchmarks are still in the same ballpark which is impressive in its own right. It will be interesting to see where this goes.