Domain availability checker, only 1 connection for whois?

Hi,
I have a questions about the domain availability checker.
I have proxies load and I use it with 50 connections in the settings.
The IP check went quickly with 50 connections from start to finish.
But when it gets to the Whois part, it starts at around 15 connections, then number of connection drops down slowly till only 1 connection.

How can I set more whois connections?

1 connection makes thing really slow when checking several thousand domains.

Thanks