Google's mod_pagespeed and Cloudflare have no known conflicts at this time. Cloudflare's CDN
and optimization services will generally be faster because Cloudflare is a CDN (content delivery network)
as well, so running mod_pagespeed will likely be somewhat redundant. Many of the features offered by mod_pagespeed are also already available in Cloudflare's feature set, with nothing to install on your
server at all.
Related: PageSpeed DNS records