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Thanks Ryan!

That all makes sense. Really appreciate it

Thanks Ryan.

I've now added in a cache date on all images in W3 Total Cache.  I will also add in image height and with attributes for all images in my theme.  All the images in our posts already have those attributes. Hopefully the theme files will help.

I've installed w3 total cache a few weeks ago, but our site speed still seems incredibly slow for some people. It hasn't improved noticeably.

I've checked in "Page Speed" and the score is 82/100.

My page speed results are here, showing a speed of 40 seconds. I get an F for compress text, compress images and cache static content.How do I resolve this?

Also..

I've set up a CDN.  Is it a problem for SEO, that there is a copy of all my images and content at cdn.mysite.com? We get lots of search traffic through google images search.