Significant Mbps drop through router

[#1]

Just changed over to fibre on a contract which guarantees 9.6 Mbps download speed. At the outset we experienced a lower speed so a BT engineer visit was arranged. The engineer investigated the roadside cabinet and the line into the property. The speed improved slightly but not significantly. He replaced my old standard socket box (and there is only one at the address) with a chunkier brand new BT box. On testing he recorded 13+ Mbps at the socket but immediately he connected the brand new Plusnet Hub One Sagecom router the download speed dropped to 8/9 Mbps whether wireless to a laptop or connected via ethernet cable. 

The engineer tried the connection with a new BT router which produced identical results. 

Can anyone explain why there is a drop of 5 to 6 Mbps when connecting to the router with the standard new 36" cable ?

Thanks...

[#2]

Hello @Celloheaven

Thanks for getting in touch and we are sorry to hear you are experiencing a problem with your speeds. I can see we have arranged an engineer to investigate this today, and our faults team will update you here, once we know more. 

[#3 · Accepted solution]

Thanks for the reply and yes I'm awaiting the engineer as I write but the reason I threw the question out there was because it'll probably be the same engineer as yesterday at which time he was unable to answer the question. However, whilst at my address he called the Plusnet tech dept who told him he had fulfilled his obligation in ensuring a minimum 5.8 speed to my socket. I suggested that figure related to ADSL (not to my fibre contract which is a guaranteed 9.6) but the BT engineer dismissed that. I'll be interested in his approach today.

So to update : the Openreach engineers have today worked primarily at the pole near the address and have successfully increased my download speed to 11.5/11.8 which surpasses the 9.6 guaranteed by Plusnet. A job well done. 

However...the unanswered question of why the speed dropped dramatically when router was connected remains. The drop in speed is now less dramatic (13+Mbps to 11.8 instead of 13+Mbps to 8.5 before) and there is chat in various forums about particular routers behaving this way but nothing specific. We've tried the same exercise with 3 brand new routers and they are all reducing the speed. Notwithstanding this issue I have to say that I'm happy with our achieved speed after all the work done...and very promptly so much appreciated. Thanks to both Plusnet and Openreach.