Switching to 5GHz on the mobile hotspot, I saw download speeds jump.
So what gives? After confirming with Samsung and the carriers that nobody was capping speeds, and checking out other devices including the OnePlus 7 Pro ( $400 at Amazon) and LG V50 ThinQ ( $319 at Amazon) 5G, it appeared that the issue lies in an Android setting that has Wi-Fi hotspots broadcast Wi-Fi over a slower 2.4GHz frequency, instead of the much faster 5GHz. While the Samsung Galaxy S10 5G on Sprint pulled in fairly consistent download speeds between 100 and 200Mbps throughout the day (and an S10 5G on Verizon getting between 400Mbps and 800Mbps) the iPhone was having a hard time getting past 30Mbps. Testing out Sprint and Verizon's 5G networks in Chicago in 2019, I was curious to see what would happen when sharing the speedy connections with a 4G-limited iPhone over Wi-Fi. But if you have an Android phone, you might not be getting the fastest speeds you can, which means that websites on your laptop or tablet, for example, load much slower than they could. Using your phone as a hotspot gives you an internet connection anywhere, and one that's far more secure than using free public Wi-Fi. Changing a simple setting could let you better share your phone's 4G or 5G connection with others.