jacorb hits a limit when returning a heavy payload

Issue

A corba client calls a POA deployed into a 5.1.0 EAP server. The service called just returns a list of pojos. Each pojo contains around 15 fields. I find that, with a given sample, returning any number of pojos between 0 and 29100 gives back the result in less than 300 ms. But after that magic number, the performances start decreasing exponentially (eg: 12844 for 29200, 126471 for 29300).
This behavior is surprising, given that the complete sample, when serialized onto the disk, takes less than 9 Mb. The sudden decrease is also very much unexpected.

Is there any recommended tuning?