I have this code to concate some array elements:

StringBuilder sb = new StringBuilder();
private RatedMessage joinMessage(int step, boolean isresult) {
        sb.delete(0, sb.length());
        for (int i = 0; i <= step; i++) {
            if (mStack[i] == null)
                continue;
            rm = mStack[i].getCurrentMsg();
            if (rm == null || rm.msg.length() == 0)
                continue;
            if (sb.length() != 0) {
                sb.append(", ");
            }
            sb.append(rm.msg);
        }
        return sb.toString();
    }

Important the array holds max 10 items, so it's not quite much.

My trace output tells me this method is called 18864 times, 16% of the runtime was spent in this method. Can I optimize more?