@KhurramRafiq 

Disconnect the Deco and connect your computer directly into the modem, and do the same test. You want to make sure the slowdown is not coming from the ISP.

Drops in speed with respect to the router are generally seen over time. For example, in the morning all speeds are good, then a couple of hours later, every device is slow, and rebooting the router restores it. This issue of a download dropping right then and there is different.

@Tony 

Thanks for your reply.

I am connected with my VIRGIN wireless (no need to connect wire) - NOT getting same issue at all, but speed remain same regardless what I am downloading. that is the reason I have DECO M4 so I can get full speed in my room.

it is only when I am connected with DECO I experience this issue.

I havn't set any bandwidth for any device in DECO, only 2 devices connected.

See the picture when downloading with DECO and VM (respectively) 

and If I don't any downloads it goes right at 100mbps all the times...

@KhurramRafiq 

Previously I was mentioning to take the actual cable from the main Deco and plug that into your computer so in this case, you will need to wire.

@Tony 

Thanks for your comments.

I havn't tried that - but surely will give it a try - need to find another ethernet cable.