This should be the translation:

    for d in *
        test -d $d
        or continue
        begin
            cd $d
            time-consuming-task-1
            time-consuming-task-2
        end &
    end

However it errors; seems you can't do `end &`. Probably a bug; I'll report it. When fixed, I imagine we could actually do it even better with fish:

    for d in *
        if test -d $d
            cd $d
            time-consuming-task-1
            time-consuming-task-2
        end &
    end