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.