scribu

This plugin improves the gallery template by making the following tweaks:
<ol>
<li>Puts all the gallery css in a separte file, for easy customisation and faster page load.</li>
<li>Adds an id attribute to the code, so that two or more galleries can have a different number of columns and still display correctly on the same page.</li>
</ol>
Links: <a href="http://scribu.net/wordpress/improved-gallery" rel="nofollow ugc">Plugin News</a> | <a href="http://scribu.net" rel="nofollow ugc">Author’s Site</a>

Improved Gallery

Improves the built-in gallery in WordPress 2.5 and newer.

Daryll Doyle

This plugin enables Parallelized Downloads / Domain Sharding for your WordPress images.
This is a good alternative if you don’t want to pay for a CDN but have an image heavy site.
Domain sharding is a technique used to increase the amount of simultaneously downloaded resources for a particular website by using multiple domains. 
This allows websites to be delivered faster to users as they do not have to wait for the previous set of resources to be downloaded before beginning the next set.
This plugin is only useful if you have an image heavy site. For sites with very few images you may see an increase in load times with this plugin due to increased numbers of DNS lookups.
See <a href="https://www.keycdn.com/support/domain-sharding/" rel="nofollow ugc">here</a> for more information on Domain Sharding

Parallelize Downloads

Enable Parallelized Downloads / Domain Sharding for your WordPress images.

Improved Gallery vs Parallelize Downloads