If you mean that a huge list is to be sent to a single client, I confirm that the delivery operation is sequential and involves a single thread of the pump pool.
This is not the typical usage and, for instance, I confirm that you have to enlarge <max_buffer_size> and <sendbuf>, whose defaults are for an intermittent update flow for many clients.
Are the ADD events supplied by the Data Adapter marked as part of the snapshot or as real-time updates?

Anyway, a CPU bottleneck was not expected.
Please specify the build of the Server related with the reported class names. We cannot find them in the latest 6.0 beta.
Could you provide more information? for instance, a screenshot from the Visual VM ?