My assumption is also that something on his network is compromised, and getting his IP into reputation issues. Tarpitting (serving content slowly from the edge

My assumption is also that something on his network is compromised, and getting his IP into reputation issues. Tarpitting (serving content slowly from the edge, in order to slow down bots) is necessarily one of the most expensive tools in a WAF/CDN's toolbox. It's much more likely that something on his network is sending sketchy traffic to CF-fronted/Google sites, and the slow loading he's experiencing elsewhere is because his upstream is being saturated by whatever is happening on his network.