The implementation was slow/inefficient. This is really silly for
_performance_ counters. And contrary to the other statistics, nothing
actually reads them, except for debug logs.

If you really want debug-only performance timers, use this:

    mtime_t start, end;

    start = mdate();
    compute_decimals_of_Pi(100);
    end = mdate();
    msg_Dbg(obj, "spent %"PRIu64" us computing", end - start);