notorioushttp

Allows developers to log variables, see the call stack, benchmark code; all to a logfile, so your page doesn’t get polluted with log messages. Each log message records the date and time, IP address, requested page, and file and line the log message came from, as well as the information to be logged.
The plugin has an options page which allows you to randomize the cookie and logfile, for security purposes; as well as turning logging on and off for a given user. (Via cookie.)
Documentation for the aaLog class can be found at the <a href="http://www.notoriouswebmaster.com/2013/07/01/aalog-a-logging-class-for-php/" rel="nofollow ugc">NotoriousWebmaster blog</a>.
Documentation for the aaLogWP plugin, which includes the aaLog class, can also be found at <a href="http://www.notoriouswebmaster.com/aalogwp-documentation/" rel="nofollow ugc">NotoriousWebmaster blog</a>, though on a different page.

aaLogWP

Allows developers to log variables, see the call stack, and benchmark code; all to a logfile, so your page won't get polluted with log messages.

The MO Cache provides simple and fast caching of the translation MO files using the <a href="https://codex.wordpress.org/Class_Reference/WP_Object_Cache" rel="nofollow ugc">WP Object Cache</a> mechanism. 
This plugin is necessary to be used <a href="https://codex.wordpress.org/Class_Reference/WP_Object_Cache#Persistent_Cache_Plugins" rel="nofollow ugc">persistent caching plugin</a>, otherwise you can not benefit from this plugin.
For localized WordPress, which was newly installed, the loading time of translation files accounts for 70% of the entire processing time. 
You can make this process 3 times faster by just installing this plugin.
<a href="http://m4i.hatenablog.com/entry/2011/12/10/000407" rel="nofollow ugc">Description in Japanese （日本語での紹介記事）</a>
<h4>Features</h4>
<ul>
<li>No configuration</li>
<li>Supports multiple languages</li>
<li>Supports plugin’s translation files caching</li>
<li>Supports WordPress Network/Multisite installation</li>
<li>A simple plugin in less than 100 lines</li>
</ul>

MO Cache

Simple and Perfect solution.

Improving the site performance by caching translation files using the WordPress standard cache mechanism.

aaLogWP vs MO Cache