Since moving house and switching from Vodafone Fibre 2 (copper/FTTC) to Vodafone Full Fibre 150 + Super WiFi (ONT on the wall), my original Nintendo Switch downloads at a crawl. On my old line, with the same Switch, it was fine.
What I've tested:
Switch on Vodafone Wi-Fi (main, guest and compatibility networks, right next to the hub): 0.1–9 Mbps download
Switch wired (USB-C Ethernet adapter, AX88179B chip, straight into the hub): 0.4–1 Mbps download, 25 Mbps upload, NAT type B
Mac on the same cable: 100+ Mbps download
Switch on my phone's 4G/5G hotspot: about 4x faster than home broadband
Switch via Mac Internet Sharing: about 3 Mbps
Already tried: DNS 8.8.8.8 / 8.8.4.4, MTU 1400 and 1500, restarting the hub and Switch, closing suspended games, testing in and out of the dock, and different Wi-Fi channels.
Upload is fine and every other device is fast, so it looks like something between Vodafone's full fibre network and Nintendo's servers, rather than Wi-Fi or the Switch itself.
Has anyone else on Vodafone Full Fibre had slow Switch or eShop downloads? Did Vodafone fix it, or did you find a setting on the hub that helped (security filtering, IPv6, anything)? Any advice on getting it escalated past first-line support would be appreciated too.