Nitpick just because.

Orders by hour could be made faster. The issue with it is it's using a map when an array works both faster and just fine.

On top of that, the map boxes the "hour" which is undesirable.

This is how I'd write it

    long[] ordersByHour = new long[24];
    var deafultTimezone = ZoneId.systemDefault();
    for (Order order : orders) {
        int hour = order.timestamp().atZone(deafultTimezone).getHour();
        ordersByHour[hour]++;
    }

If you know the bound of an array, it's not large, and you are directly indexing in it, you really can't do any better performance wise.

It's also not less readable, just less familiar as Java devs don't tend to use arrays that much.