@Sulvera 

Go into your QoS settings and make sure the speed there is set to reflect your internet speed and not slower.

Something may now be getting in the way of the Deco signal, so try and move them closer or to an area of the least amount of obstacles. 

Another thing to try is to swap one of the satellite units with the main Deco.

@Tony 

Thanks for the reply Tony. I did another speed test before trying any of your suggestions. The speed got 330 mb/s. 

I am happy now but I'm not sure what causing the slow down in speed.

Qos setting has the bandwidth set at 933mb/s. Its also set as standard with no device set as preferred. 

I doubt its the placements of the deco as my masterbedroom has the master deco in the same room. And the night before I was having speed issues. (I will be keeping the master in my room for obvious reasons so I cannot do your suggestion about switching the slaves with the master).

I doubt its obstacles, all deco are wall mounted.

I guess I will keep an eye on the decos now that they are working. 

@Sulvera I have 3 M4's and tried your recommendation re QOS but no joy.

I have 1 Gbs service but am only getting around 110 Mbs wifi even when sitting beside the main Deco unit.

@Tony 

Hi Tony, would you have any other suggestions for improving the wifi speed? Over the holidays I've been getting constant dips in wifi speeds, can't even watch a youtube video @360p without it freezing every minute because of the weak connection/slow speeds.

@Sulvera 

I would get a hold of our support team at this point. They will be able to narrow it down for you to see what else to try.

@Tony 

Getting very slow WiFi speeds from deco m5 - one of the worst thing I have purchased I my life, please don't buy this product.