Very slow email accepting with single connection?

[W1zzardTPU]

We're using ZoneMTA as internal mail router to distribute mail to various external services or deliver directly.

I noticed that when we try to send a lot of mail through a single connection, the mails are accepted very slowly, like a few per second only.

Is this a configuration issue? I tried increasing the various parallelisms within ZoneMTA, but no luck.

Making the sender-side multi-threaded is always (a complicated) option, so I'd rather prefer to fix this within ZoneMTA.