I tried out Cloudflare’s 18.104.22.168 DNS resolver for iOS and it does seem to improve the speed of my mobile web browsing. The app looks up internet addresses more quickly and more privately than your ISP’s DNS.
Likely due to iOS design and limitations, Cloudflare 14 must run as a VPN service on iOS. So, when you want to run your iOS VPN, you have to turn off Cloudflare. For those that run VPN’s all the time, this app won’t work for you. For those that run VPN’s on demand, this may be an option.
I’ve been using it a few days and my phone browsing does seem faster.