Any Idea why it is a problem, router cpu, memory, firmware? ...
It is hardware limitation not software limitation.
Personally, not aware of any consumer routers that will maintain speeds when features such as QoS is enabled.
Consumer routers use some variation of Cut-through switching to get those high speeds. All those that I've come across, the CTS can not function when features such as QoS is enabled.
You can post another topic and see if there are any members here who know of any model(s) that can handle CTS and QoS together.
If you are technically inclined and have a spare computer system lying around,
You can build your own router.
Thanks apvm for the guide I have come across and actually used it .
casualttester; on mentioning about CTS and consumer routers and also reading further about it, I did go back and try out my previous router (asus ac66u) with QoS and the results were the same, maximum 10-15 % of the original speed.
I did look up more on QoS router issues, seems to be common trend across all brands, actually Netgear suggests if speeds are more that 100Mbps, one should no longer need to enable QoS.
Seems to be a good suggestion to go and setup my own build router box and a pre-build router img as base, now that I am seeing more and more limitations of router hardware and requiring me to invest every 4-5 yrs or so in them.
Thanks all again for quick and appropriate replies.