what is the difference between edge certificate and origin certificate?
Edge certificates are provided by CloudFlare and shown to your visitors. They will encrypt traffic between your visitors and CF.
Origin is a certificate on your server and needed to encrypt traffic between your server and CloudFlare
Can cloudflare help me with increasing the site speed?
Of course. By caching static content for example. Or Minfying JS, HTTP2, Brotli, Rocket Loader…