I know this might not be the solution for you, and I hate it when people suggest this as its usually stupid! I started Xcode XIP and was stuck at 600kkbps, out of massive frustration and a last ditch effort, it plugged my MBP into the lan, turned off wifi and restarted the download, sat at 60MBPS and finished in under 5 mins. I wonder if its a packet size thing? Perhaps give it a go. for fun I even tried it from the App Store. Same thing its taken under 5 minutes to download hardwired to the LAN. For reference my wifi is a unifi setup with ap pros Ac, so is pretty good and I can get full bandwidth over it most of the time.