Update configuration.md +performance, batch size

[MaximilianKohler]

I don't know if this is correct, but there's no info in the docs about these kinds of settings. All I could find was one comment here #172 with some info.

I'm trying to figure out a solution to #1717. I'm hoping that the larger the batch size, the longer the delay between retries will be, but I'm not sure if that's true.

I saw another "performance" section https://listmonk.app/docs/maintenance/performance/, but it's under "maintenance" so this seems more appropriate under "configuration".

[knadh]

The campaign stats use this metric to show an approximation of progress. By setting a lower number, the progress accuracy can increase.

This is in correct. As of last version, every message is tracked granularly showing accurate progress. Batch size has no effect on the rate calculation.

Under Settings -> SMTP -> retries, listmonk will attempt to retry any failed deliveries after the full batch has been sent. So increasing the batch size will increase the time between retries. This can be helpful if you're getting 4xx sending errors.

This is also not correct. When a message is sent, if it fails, it's retried on a new connection then and there, before being marked as an error (if the retry also fails). There is no batching in retries.

[MaximilianKohler]

So batch size had a minor role in previous versions but is largely meaningless now?

[knadh]

It comes in handy when working with a large number of subscribers. Pulling bigger batches (which'll use more memory, but reduce the number of DB queries) will help achieve more throughput.