@Anthonyb22 

Results of speed test are dependent on geographical and networking location of speed test servers. It is possible that when you don't use VPN, for some reason an app chooses server too far, but when VPN is in use, speed test server sits right next door to VPN server, figuratively speaking.

For Satellite Decos, what important is proper placement. Where Satellite Deco is located may have substantial impact on speed of wireless devices connected to it. You did not clarify that, but I am assuming your Satellite Decos are not wired. When they communicate wirelessly, usually Satellite Deco is connected to Main Deco. The quality of signal between them must be good, otherwise speed on device connected to such Satellite Deco will suffer, even if device reports strong connection to Satellite Deco.

You can use Deco app for a quick check if each of your Satellite Deco is properly placed in your house. Touch Globe with Internet word under it on Deco app main page, it will show list of all Deco units in your Deco mesh. Touch Satellite Deco in that list and its information screen will open. Check what it says under Signal Source, and here is example from my Satellite M9:

It says this Satellite M9 is connected to Deco named Living Room, which is my Main Deco, and that signal between them is excellent. Three blue bars may not be always achievable, but you should see at least two blue bars and all three WiFi bands, one 2.4GHz and two 5GHz. Anything less than that means Satellite Deco must be moved closer to the Deco it is connected to, to improve speeds.

@Alexandre. 
 

Hello Alexander thank you for your reply really appreciate it. So my main deco is connected to my virgin hub3 by the Ethernet lead that's provided when you Buy the M9. And my other 2 decos are just wireless connected by the power only. I have done as you said and looked at the signal. My main Deco that's connected to my Hub 3 which is in modem mode is full on 500MB, then my second deco is down stairs in the living room literally 1 floor below the main deco the signal strength is on 2 bars but noticed sometimes it bounces to 1 bar and back to 2 no idea why with them not being far apart at all but my speed test on the second deco is around 280-300. Then my third deco is in another room in my house there is a 16ft gap then a seperate room we're I couldn't get my virgin internet to reach normally. This deco is getting speeds between 40-60MB today and had only got 1 bar signal everywere I move the deco in the room it's only capable of picking up 1 bar. I didn't think a 16ft gap between my second and third deco would show this much of a bad result. I'm still confused about the VPN so when I do a speed test connected just to the deco I get 40-60 but with the VPN on I'm getting 110MB result. The speed test is showing it's connected to a server in Manchester so I also connect my vpn to a Manchester server and it's a lot faster so the speed test are coming back from the same city with better results from the VPN. I have my fast Roaming and Beamforming both enabled and my operation mode is in router mode on the deco app. Im not sure if I should try it in access point mode I'm using the same wifi credentials that my internet provider gave to me. Don't really understand the difference between router and access point mode thank you very much for taking the time to help me 

@Anthonyb22 

In regard to Access Point vs. Router mode, here is formal TP-Link document: What’s the difference between Access Point mode and Router mode on the Deco?

Informally speaking, Access Point mode is always preferred, unless you need content filtering or traffic throttling. Also, Access Point mode is great for troubleshooting, because it removes all these traffic throttling settings, and helps to verify if they were incorrect or something else.

Important: for Deco to function in Access Point mode you must run your Hub3 as a router. 

I would recommend to try that: turn Hub3 back to router mode, switch Deco to Access Point mode and run speed tests again. If you don't miss any of Deco router mode features, keep it that way.

As for placement of Satellite Deco units: ideal placement is 15-30 feet from Deco they are connected to (usually Main Deco). Also, if there is a room where you want to have good Internet speeds, you must place Satellite Deco approximately in the middle between that room and Main Deco, not in the room. If you can.

This is the problem with WiFi mesh marketing: they don't tell you how important is proper Satellite units placement.