From what I found when I was just starting to figure out proxies, they are slow if the provider resold resources, the channel is overloaded, or the route to the server is bad. All that is complicated and there is no real point in digging into the reasons. Just choose a good provider. I have FloppyData, and from them I also found recommendations for top anti-detect browsers https://floppydata.com/systems/macos/ for macOS. Overall, I am happy with how it works now. And I have not seen slow proxies for a long time.