NS-700 eats up bandwidth when connected to DD-WRT router

[Stealth1007]

I recently installed dd-wrt firmware on a Linksys E2500 router. The download speed is steady at 60 Mbps when a laptop is connected. However, as soon as I connect my Panasonic NS-700 phone system the download speed goes down to 9 Mbps.
This also happens on a Buffalo router which has factory installed dd-wrt firmware.
Then I run a TP Link router, the bandwidth stays at 60 Mbps.
Has anyone seen this problem before?

thanks

 

[Billxx]

I have a NS700 here in my lab.
I also have a Buffalo Factory DD-WRT Router for all my network connections (NS700 included)

I ran a speed test (Ookla) with and without the NS700 connected and had the same speed for Up and Downloads.

I also connected to the web browser of the NS700 and no change in speed.

My NS700 is running firmware 004.41017

Hope this helps in some way...