There are quite a number of comments on forums and twitter from people who claim that since the networks introduced 4G data speeds, the existing 3G service has suffered a drop in speed.
This could be a knock on affect of the 4G bandwidth or it might be the networks finding a way to make people want to move to 4G data tariffs.
If you have 4G in your area, have you noticed slower 3G data speeds?
Gavin's Gadgets & Photography 
Yes massively. Gone from 20 mbps to 4.25. I really don’t like EE.
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That’s a huge difference. Clearly they are trying to push people to paying more with their 4G contracts.
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I managed to get T-Mobile to confirm that they now limit download speeds on their 3G network to around 4mbps (but in Bristol I found I can still get 10mbps for some reason). My 4G SIM in the same area as my 3G SIM on the same 3G network can download at 20mpbs! So EE/T-mobile actually set limits for 3G.
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Iv found the opposite Gavin with Vodafone I can now get 3G in places I could only previously get 2g
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