$ time go test -failfast -short -run 'ΔBTail/rebuild' >x

before:

        real    0m17,852s
        user    0m23,068s
        sys     0m1,825s

after:

        real    0m9,622s
        user    0m12,515s
        sys     0m1,274s

        $ time go test -failfast -short -run 'ΔBTail/rebuild' >x

before:

        real    0m43,071s
        user    0m57,487s
        sys     0m4,869s

after:

        real    0m17,852s
        user    0m23,068s
        sys     0m1,825s