@Martian 

M9 Plus is the best Deco hardware TP-Link offers for WiFi5 networks. That hardware provides superior speed/coverage benefits for higher cost per unit. M3W is on the opposite side of that. 

M3W works as WiFi repeater. You can google "How WiFi repeater works" to get familiar with technology, its benefits and shortcomings. In summary, if placed properly WiFi repeater will improve WiFi signal and extend its range, but you are paying for that with the reduction of speed. Yes, you'll have stronger signal, but with slower speed.

Practically, it works in situation where someone chooses between "slower WiFi with repeater or no WiFi at all."

The placement of M3W (like any WiFi repeater) is also important. It can not make WiFi speed faster than it gets from Deco it is connected to.

Here is simple test you can run for your current M3W placement. Note which Deco it is connected to, Main or Satellite. Turn M3W off (unplug it), and standing with your mobile exactly where M3W is, force mobile connect to same Deco unit. Ideally, place mobile right where M3W was, perhaps by covering electric outlet to which M3W was plugged with your mobile.

Then, run speed test on mobile. What you will get will also be maximum WiFi speed M3W gets from Deco in that location. Then, divide that by half. That would be approximate speed you should expect on mobile device connected to M3W.

Options you have:

1. Find different place for M3W, ideally in the middle between Deco it connects to and mobile devices or room it will be serving. It might improve speed on mobile connected to M3W, but don't expect dramatic improvement.

2. Retun M3W and accept WiFi speed you have at "slow Internet speed spot". 170Mbps down is really not that bad, you could stream up to 4K on that speed with no issues.

3. Get additional Deco M9 instead of M3W, but it is more expensive and hard to get in a single unit set, usually sold as 2-pack and 3-pack.

If you have Ethernet cable or TV cable terminating near "slow Internet speed spot," I can share with you more options to consider.

@Alexandre. 

Thanks for your explanation in depth.

It is indeed a surprise to find that the speed of M3W can only achieve max 1/2 of that of the M9 Plus can achieve at the same spot.

It's 1/3 in my case - what a shame - I decide I will return it.

I think it should be detailed in the product specs but the specs simply mentions and advertises with the max speed as: up to 867 Mbps - 5 GHz and up to 300 Mbps - 2.4 GHz

Actually it's sort of misleading - given the speed test in my mobile shows 170Mbps via M9 plus (that is still well below 867Mbps) - why it is downgraded by 1/3 to 52Mbps after M3W instead?

Having said that, I have to admit the Deco M9 Plus is a neat product.

I will get an extra M9 plus if needed.

Thanks for your fine support!