Unexpected delay before message processing in EventGridTrigger

#### Is your question related to a specific version? If so, please specify:
```xml
netcoreapp3.1
v3

```

#### What language does your question apply to? (e.g. C#, JavaScript, Java, All)
C#

#### Question
I see that the code execution time inside the Azure Function is 2s, but event handler shows that the total time is a few minutes
Why do some messages from EventGrid are waiting so long before start actual processing?

```cs
public class Handler
{
[FunctionName("Handler")]
[Singleton]
public async Task Run([EventGridTrigger] EventGridEvent eventGridEvent, ILogger log)
{
log.LogInformation("Event:");
}
}
```


Start with the EventGridTrigger binding and the [Singleton] attribute in Handler.Run, then compare the reported two-second function execution with the event-handler timestamps. Determine where the queueing delay occurs and document the cause or required configuration; the issue is done when the delay is explained and a reproducible resolution is identified.

Written by the indexing model from the issue text.