It’s true that some CDNs make SPDY/full SSL sites even slower, but it depends on a lot of things. We always recommend clients test and see if it’s worth adding CDN+SPDY to their sites or if SPDY in itself is enough. Usually it is.
After the first handshake the client (browser) gets all the material in one swoop instead of being limited to 2-6 parallel downloads at the same time (that’s an HTTP limit that modern browsers circumvent but only up to a limit). On a site with 120 requests (from the same domain) that’s 20 round trips, at least. SPDY solves that and HTTP2 (when it comes out) will solve this problem as well.