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Work with WP-Cache or WP Super Cache?

Yep, WP Widget Cache is a plus for them, as it reducing the cost for caching a new page

Want to be sure it really reduce your server usage?

View the source of your site page, there maybe some code like this at the foot:

<!-- ** queries. ** seconds. -->

If not, please put these codes in your footer template:

<!-- <?php echo get_num_queries(); ?> queries. <?php timer_stop(1); ?> seconds. -->

After using the WP Widget Cache, I think you'll find the number of 'queries' reducing a lot. Of course, that will depend on how many and what widgets you use, some sites can gain more than 70% improvement.

How to know it works?

You can have a look at the source of the web page, and search

<!--WP Widget Cache End -->