I'm sorry that you have found giffgaff's service no better than the one you moved from.
You mention that the 3 service you were on had only recently "became so interminably abysmally slow".
I wonder, before you moved, did you manage to find out what was behind this (from 3's customer service) & if it was generally the case in your area (from other 3 users in your general location).
While it is true that giffgaff (using O2's network) is rarely faster than other networks, for many, it 'just works' at a reasonable level.
Before you move to another network or give up on technology altogether, if you have not already done so, do ask around locally. If yours was the only phone on 3 that experienced a sudden drop in performance and you find other giffgaff/O2 users who have 'usable' data then perhaps there is some issue with your phone.
Please Note: I don't usually assume a phone is at fault, but in this case, the O/p experienced a sudden drop in service on their previous network, so it is possible something other than giffgaff's network is at fault.
Also, it is my understanding that "Traffic Flow applies to the £20 goodybag. Speed restrictions apply to our Always On data after 6GB of data usage." applies whatever method is used, so changing to 3G will make no difference to these restrictions.
@matt0941 unfortunately matey it's not for every area ...that are still real bad spots even with alleged 4g ...it's great in Liverpool and the Wirral imho ....but we are lucky up here .
Hope your next choice of provider works better for you .
I live in liverpool and work on the wirral, this is not the case. Its poor
EE has the fastest data speeds, with Three nudging into second place. Both also have good 4G coverage compared with the other networks. Really, O2 shold have the best 4G coverage as it was the only network to commit to 98% population coverage.
Still, 5G is round the corner, and that will save the day. Or so the networks would have you believe.