Mosquitto client consuming is slowing down and data gap increasing - .net-core

Question

In a system with multiple devices, diagnostic data is published to separate topics for each device to Mosquitto Broker. A client application tries to consume data by subscribing to these topics separately for each device. The client receives 5 or 6 record per device per minute.
My problem is, after worker application started, the data received from Mosquitto Boroker starts to have delays on the client side. These delays start from 1 second, as time progresses, it lags behind by 1 day and 2 days during the time it is on. After stopping borker, the client continues to consume data.
Any ideas on what causes these delays?
protected override async Task ExecuteAsync(CancellationToken stoppingToken)
{
string dataHost = _configuration.GetValue<string>("DataHost");
mqttClient = new MqttClient(dataHost);
mqttClient.MqttMsgPublishReceived += MqttClient_MqttMsgPublishReceived;
var list = _configuration.GetValue<string>("DeviceList").Split(',').ToList();
foreach (var deviceId in list)
{
mqttClient.Subscribe(
new string[] { $"{deviceId}/#" },
new byte[] { MqttMsgBase.QOS_LEVEL_AT_LEAST_ONCE });
}
mqttClient.ConnectionClosed += MqttClient_ConnectionClosed;
mqttClient.Connect("client", "username", "pass", false, 10);
}
private void MqttClient_MqttMsgPublishReceived(object sender, MqttMsgPublishEventArgs e)
{
try
{
var message = Encoding.UTF8.GetString(e.Message);
var messageArray = e.Topic.Split('/');
if (IsValidJson(message))
{
ItemValue itemValue = JsonConvert.DeserializeObject<ItemValue>(message);
_diagnosticRepository.InsertAsync(new DiagnosticDTO
{
value = messageArray[4].ToString(),
message = message,
messageTime = DateTime.Now,
deviceTime = itemValue.datetime,
sequenceNo = itemValue.sequenceNo,
});
}
}
catch (Exception ex)
{
_logger.LogError(ex.Message, "Consumer Insert Error");
}