As giffgaff is a subsidiary of O2, you can the same fast speeds from O2. In OpenSignal’s September 2021 report, giffgaff via O2 was the second fastest 5G network, with average download speeds of 91.2 Mbps. As for 4G, average download speeds clocked in at 17.1 Mbps, with average upload speeds of 5.4 Mbps.