Connection settings for Asus RT-AC87U?

[#1]

I have replaced my  Plusnet Technicolor T6582n with an Asus RT-AC87U to benefit from a faster more reliable router but I have noticed that I get a better WAN performance with the old Plusnet Technicolor T6582n router than the new Asus router. The Asus router achieves download speed typically 54Mbps vs 67Mbps for the T6582n and the upload speed differential is even more marked: 7Mbps vs 19 Mbps.

Can anyone suggest why this might be? 

Has anyone tried changing the MTU, MRU or other settings to improve speed on an Asus router?

[#2]

 But how are you connecting the Asus router to the broadband as it doesn't have a modem?  Your download/upload speed will be due to the modem not the router.

[#3]

It's connected via the BT Openreach modem - exactly the same as with the Technicolor router. So I don't think that's the issue.

Moderator's note by Mike (Mav): Full quote of preceding post removed as per Forum rules.

[#4]

Are you testing with a wired connection - there shouldn't be any different in wired performance between two routers connected to the same modem.

[#5]

Yes.  I've tested it for a week with a single wired ethernet connection and the same cable to eliminate any other possible conflicts.  I agree there should not be any difference but I have tested very carefully and there is a very clear difference. So there must be some explanation.

Moderator's note by Mike (Mav): Full quote of preceding post removed as per Forum rules.

[#6 · Accepted solution]

I have found the solution!  It was the QoS settings in the router, which were set to manual and limiting the download and upload speeds.  I have reset to automatic and the speeds have now jumped up to what I was expecting.  Problem solved.