A cloud server (Digital Ocean droplet with Cloudflare CDN) runs a Wordpress website that hosts a software download for its visitors. One day, users report that their download only runs with 30kb/s, where before it ran easily with >5mb/s. A first check in your browser shows the site to run smoothly, but the download is reproducably slow. Via FTP, the download is fast.
Which steps should I take to track and fix this behaviour?