Here's a parallel implementation that gets me 950 commits per second:

    manycommits() {
        h=@; t=`git rev-parse @^{tree}`
        for i in `seq 2 "$1"`; do
            h=`echo "$*" | git commit-tree $t -p $h`
        done
        echo $h
    }
    twice() { "$@" a & "$@" b ; wait; }
    twice twice twice twice manycommits 10000 | while read h; do git merge --no-edit $h; done