You can use the Cloudflare with WP Fastest Cache. In order to minimize potential conflicts, you need to disable the optimization options of the Cloudflare.
WP Fastest Cache is totally compatible with Cloudflare. In order to minimize potential conflicts, you need to Cloudflare cdn integration.
How to Find API Key
1. Go to https://www.cloudflare.com/a/profile
2. Click on “API Tokens” tab
3. Click on “View” and copy the Global API key