Speed Kit is built on Service Workers, a new technology that is currently entering all major web browsers. Since Service Workers can only be enabled on SSL-secured websites (HTTPS), Speed Kit is also only available when SSL is turned on.
Okay, but why are Service Workers only available on SSL-secured websites then?
SSL-encrypted web communication (HTTPS) is considered preferable over non-encrypted communication (HTTP), because it protects sensitive information exchanged between client and server.
In an effort to accelerate the adoption of HTTPS throughout the web, new browser features are often only made available for SSL-secured websites. Like HTTP/2 and many other features, Service Workers can therefore only be used when SSL is enabled for your website.