In its most common usage, cloudflare acts as a security intermediary between your server and the front end user.
Simplified, cloudflare filters through users and their information, saves information and weeds out potential threats and then sends the users through to your website.
So, just think of cloudflare as a security filter between your server and the end users.
If your website is slowing down you will want to check out the cloudflare settings to make sure they are compatable with your server configuration.
A common misconfiguration error would be setting up the cloudflare nameservers and/or DNS.
Since you want to filter users through cloudflare you need to add your nameservers on cloudflare.
Cloudflare probably walks you through this, but there could still be a misconfiguration error on the cloudflare setup or on your websites local server.
All setting up the nameservers and DNS on cloudflare does is tell the servers to direct traffic through the "security filter" that is cloudflare first. Then, once cloudflare says the traffic is secure, they are passed on to your website.
So thats where Id check first. Double check for dns and nameserver misconfiguration errors. Cloudflare should have several articles and posts explaining this entire process in detail.
Good luck to yah mate!