After adding the plugin to the panel I checked the option "Automatically Choose Network Device" in the Properties, however on each session restart this option is reset. The bug was reported to upstream. https://code.google.com/p/xfce4-netspeed-plugin/issues/detail?id=11&can=1 In version 0.3.1 this problem is fixed.
Renaming the ebuild to 0.3.1 would be enough to solve this issue.
+*xfce4-netspeed-plugin-0.3.1 (14 Apr 2015) + + 14 Apr 2015; Michał Górny <mgorny@gentoo.org> + +xfce4-netspeed-plugin-0.3.1.ebuild: + Version bump. Fixes remembering 'Automatically Choose Network Device' option, + bug #532194.