An unencypted connection is fast, cacheable, and secure enough when you're just transfering photos and cat videos.
As far as I know, my browser does cache content served over https exactly the same as served over http.
But your ISP cannot cache said content. Say you have a classroom full of children all reading the same article on Wikipedia, and it's in a remote area with the only available Internet connection being a 0.13 Mbps ISDN or satellite link. With cleartext HTTP, a Squid or Polipo proxy can pull every . But with HTTPS, the proxy has to fall back to a separate CONNECT tunnel and transfer the same article 20 times unless the proxy is configured to intercept TLS, with its own root certificate in all browsers configured to use the proxy. Failure to cache in such a situation is inefficient, slow, and possibly costly if it causes the school to exceed a monthly Internet data transfer quota. (Source)