Articles mention the added complexity of FTP, having both control and data socket, randomized port, connection latency (N round trips vs 1 in HTTP IIRC)..
That said HTTP has an aura of atomic resource download, while FTP comes with :drumroll: a file system access aura. I barely never upload anything in shell through HTTP. And before the era of HTML drag n drop, it wasn't in my mind to upload through a browser over HTTP either. That changed a bit though. Still my favorite thing to upload over the web is webtorrent.
I believe it's higher. It's especially visible if transferring multiple smaller files, as each download requires a separate new connection (no data connection reuse).
I'm not sure why WebDAV is less popular. It has its oddities and implementation issues but it's nowhere close to FTP weirdness. And AFAIK WebDAV support is either built-in or readily available to install in all mainstream OSes.
