You may want to look into upgrading your Openfire server, as it's quite out of date by now (current version 3.8.2 as of writing this).
Here's a guide on how to do that: http://www.igniterealtime.org/builds/openfire/docs/latest/documentation/upgrade- guide.html
There may be improvements with the latest OF versions that reduce the network load possibly -- but in general it's best to run the latest for a lot of reasons.
To install the plugins via the Web Interface, yes your OF server would need to be able to reach out to the internet to download them. Alternatively you can download individual plugins from here http://www.igniterealtime.org/projects/openfire/plugins.jsp and drop those into your plugin directory on the server, or upload them from the Plugins page in the OF Admin Interface.
I was wrong before, it's actually the Monitoring Service plugin that you will want. http://www.igniterealtime.org/projects/openfire/plugins/monitoring.jar
The other plugin (Load Statistics) logs stats to a file.