> I will try to find out the reason. This should be somehow related to
> networking, because this is specific to local connections only.

Reason seems to be in socket default receive and send buffer sizes.
Right now I do not have a solution, but for your case buffers of 16k
give the best performance (same as InterClient). You can fix driver by
adding following lines into org.firebirdsql.jgds.GDS_Impl (line 1496):

db.socket.setReceiveBufferSize(16 * 1024);
db.socket.setSendBufferSize(16 * 1024);

Best regards,
Roman Rokytskyy