The fastestmirror plugin may be useful at some points, but it can be a pain in the ass at others. For example, I tried updating my CentOS installation today and because of the mirror caching it was downloading the updates at 30K/s. After disabling the plugin the speed from picking a different mirror jumped to 3MB/s.
To temporarily disable the plugin, add –disableplugin=fastestmirror to your yum command line. For example yum update –disableplugin=fastestmirror
To permanently disable the plugin, edit
/etc/yum/pluginconf.d/fastestmirror.conf and change enabled=1 to enabled=0