The data structure of interest here is that one `BeatmapSetInfo` can have multiple `BeatmapInfo` [children](https://github.com/ppy/osu/blob/deb108816cd17e92f5700391e65bd03e34aa7e70/osu.Game/Beatmaps/BeatmapSetInfo.cs#L34).

The run with one subscription has the subscription on the top-level collection (`BeatmapSetInfo`) and the overhead is around 4 seconds for a collection of 1982 realm objects.

The run with two subscriptions adds a subscription on the child collection (`BeatmapInfo`) with a total overhead of 74 seconds for a collection of 18215 realm objects.

(as an aside, the user-reported case in our deployed production build has 7 active subscriptions on various realm collections, and a total refresh delay of 85 seconds. i've extracted only the two most expensive subscriptions for this reproduction - the rest are filtered to GUIDs and add less overhead)

From my perspective, these object numbers are not large enough to cause concern.

We do have other linked realm models which could be adding hidden overheads due to how the data diff is done internally (`BeatmapSetInfo` has many `File`s and `BeatmapInfo` has attached `BeatmapDifficulty`, `Ruleset` and `BeatmapMetadata`). I'm not immediately sure whether these are adding overhead but could test further if required.

Looking at what is being done in this sample project, from a high level it feels like there has to be a way for this to be optimised at realm's end.

### Version

10.11.2

### What SDK flavour are you using?

Local Database only

### What type of application is this?

Other

### Client OS and version

OS agnostic

### Code snippets

```csharp
using System.Diagnostics;
using osu.Game.Beatmaps;
using Realms;

string filename = Path.Combine(Environment.CurrentDirectory, "client.realm");

var realmConfiguration = new RealmConfiguration(filename)
{
    SchemaVersion = 14,
};

List<IDisposable> subscriptions = new List<IDisposable>();

using (var realm = Realm.GetInstance(realmConfiguration))
{
    var beatmapSets = realm.All<BeatmapSetInfo>();
    var beatmaps = realm.All<BeatmapInfo>();

    RunTest(realm, beatmapSets);

    subscriptions.Add(beatmapSets.SubscribeForNotifications(Callback));

    RunTest(realm, beatmapSets);

    subscriptions.Add(beatmaps.SubscribeForNotifications(Callback2));

    RunTest(realm, beatmapSets);
}

void RunTest(Realm realm, IQueryable<BeatmapSetInfo> beatmapSets)
{
    Console.WriteLine();
    Console.WriteLine("------------");
    Console.WriteLine($"Running with {subscriptions.Count} subscriptions active");

    Refresh(realm);

    Console.WriteLine();
    Console.WriteLine("Performing arbitrary write...");
    realm.Write(() => beatmapSets.First().DeletePending = true);
    Console.WriteLine("Write completed.");

    Refresh(realm);
}

static void Refresh(Realm realm)
{
    var stopwatch = new Stopwatch();

    stopwatch.Start();
    Console.WriteLine();
    Console.WriteLine("Refresh started...");
    realm.Refresh();
    Console.WriteLine($"Refresh completed! ({stopwatch.ElapsedMilliseconds}ms)");
}

static void Callback(IRealmCollection<BeatmapSetInfo> sender, ChangeSet changes, Exception error)
{
    Console.WriteLine($"Got callback ({sender.Count} beatmaps)");
}

static void Callback2(IRealmCollection<BeatmapInfo> sender, ChangeSet changes, Exception error)
{
    Console.WriteLine($"Got callback ({sender.Count} beatmaps)");
}