Bulk insert Azure Cosmos DB

I can found some samples stating following should be bulk inserts:

var options = new CosmosClientOptions() { AllowBulkExecution = true, MaxRetryAttemptsOnRateLimitedRequests = 1000 };
Client = new CosmosClient(ConnStr, options);

public async Task AddVesselsFromJSON(List<JObject> vessels)
{
    List<Task> concurrentTasks = new List<Task>();
    foreach (var vessel in vessels)
    {
        concurrentTasks.Add(VesselContainer.UpsertItemAsync(vessel));
    }
    await Task.WhenAll(concurrentTasks);
}

I am running the code on an Azure Function (App Plan) with 10 instances. However I can see it is only around 4 inserts pr seconds. With SQL bulk insert I can do thousands a second. It does not seem like above is bulk inserting have I missed something?