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Famous Free CDN service is the [Cloudflare].
However, it did not work in the issue of compatibility of the server in this site (www.amelt.net).
So, there is a wordpress plugin [Jetpack by WordPress.com].It is provided by wordpress.com.
There is a function CDN(content delivery network) service called [Photon], that can be used free of charge to [Jetpack by WordPress.com].
Jetpack by WordPress.com
What is Photon?
In one of the functions of "Jetpack by WordPress.com" plugin, Photon is that free CDN(content delivery network).
By using the (Content Delivery Network) CDN, it can be made faster to disperse the load to cache the external server an image file.
Cloudflare(free CDN service) vs Photon
Photon is valid and speed up only for image.
However, your wordpress site will be stopped when Cloudflare server is stopped.In contrast, it does not affect your wordpress site when Photon(WordPress.com) is stopped.
In addition, whereas the starting Cloudflare secive is nessesary to change the domain name server(DNS), Photon is easy to start, just click and register to WordPress.com.
And it is possible to use combination CDN service and cache plugins such as "W3 Total Cache, because their role are different.
How to use the Jetpack plugin
1.Create an account with wordpress.com.Then log in to wordpress.com. (It can be later)
2.Install and enable [Jetpack by WordPress.com] plugin
3.Go to the menu of [Jetpack]
4.Click the [Connetct to WordPress.com]
5.Click the [Authorize Jetpack]
6.Enable the necessary functions of the Jetpack
How to use the Photon
Photon is included in one of the functions of the Jetpack.
When you enable the Photon, URL to link to image is rewritten.
Required for the operation check.
In addition, it is good to clear the cache of all,once you have enabled the Photon.
Lightbox not work when you enable Photon?
Lightbox does not work when I enable Photon in this site(www.amelt.net).
Note at that time, please refer to the article [wordpress:Lightbox doesn’t work when activate Photon of Jetpack].
Recommended plugins to speed up WordPress
Others, for plugins to speed up wordpress, please refer to the article [【2014 edition speed up plug-in】 plug-in list installed in wordpress].