I you don't leverage performance related features of a CDN (mostly cache), it's more a security layer. It won't improve performance until you get your hands dir

I you don't leverage performance related features of a CDN (mostly cache), it's more a security layer. It won't improve performance until you get your hands dirty or ask a professional to tune it for you (and maybe you did). A global DNS resolver may decrease performance, for instance it can give poor results on DNS based load balancers. Interested to know how you assess SEO impact and your findings.