@Gfaefife let's decode EE/BT language first:
Speed Guarantee 1.3 Gbps is ONLY to the point of supplied EE Super Hub Plus, and as long as this is achieved, everything is fine...
Essentially, supplied typical speed of 1.6 Gbps down, 120 Mbps up is on the ONT and whilst ONT and EE SH+ are connected via 2.5 Gbps connection, full duplex negotiated, this speed will be achievable to the point of the router itself...
BT/EE never guarantee any speed BESIDE router itself. Firstly, because they can't control your own home environment (WiFi, LAN), and secondly, because EE SH+ is only 1 Gbps on LAN ports...
That said, when You first start your EE SH+ this will utilize only an 80 Mhz channel, and this will be automatically assigned to it, and You have NO CONTROL over it...
With 80 Mhz bandwidth, you will never see anything over 1 Gbps on WiFi, more likely, due to standards, this will utilize an 864 Mhz or 1201 Mhz link between the capable device and Wifi, and in real life, your WiFi speed will be maxing somewhere at 750 Mbps down...
EE SH+ however, usually 24 hours after you fire it, will update itself, and utilize 160 Mhz channel bandwidth, most likely, utilizing channel 100 on 5Ghz WiFi...
With 160 Mhz bandwidth, compatible devices will be able to achieve a 2402 Mhz link between the WiFi router and the device. With this, You will be able to see 1.3 to 1.6 Gbps speed over the WiFi, and that's only if one device will demand a connection. In principle, with more devices sharing a connection, your speed will go down, but that's nothing surprising here...
You will NEVER see anything over 1 Gbps on LAN with supplied EE Super Hub Plus, because that's HW limitation.
Why EE is this stupid, not sure, but it's very close to misleading customers, whilst supplying a router, that is not able to provide expected speeds on both WiFi and LAN.
On other hand, not only WiFi router need to be able provide respective class of connection, but also device, You connecting into it.
Example:
My old Oppo Find X3 Neo is the only WiFi5 device, and it's able to utilize a maximum of 80 Mhz channels on a 5Ghz spectrum, with a maximum established link of 864 Mbps. This will make HW limitation and therefore, no speeds over, say 850 Mbps possible, more likely up to 800 Mbps.
My newer Xiaomi phone, on the other hand, it's a WiFi6E device, and it's able fully utilize a 160 Mhz channel, and establish a 2402 Mbps link with a WiFi router. On this device, I seen 1.3 Gbps+ with EE supplied router.
However, this router it's back in the box and in storage, not used at all. I went via route, to use my own equipment itself. Backbone it's on 2.5 Gbps LAN and my main WiFi AP it's capable of providing full of this, which means, I can see easily 1.5 to 1.7 Gbps on my EE BB now over the wifi, and the same over the LAN.
Obviously, whilst I have around 50 devices of different types and speed limits on my network, the usual connection speed it's in the range of 1.3 to 1.5 Gbps for me, and in peak times, my most demanding devices share this and are still capable to have approx 500-700 Mbps each of them in the same time. I am happy with that...
