Hi, sorry my last post didn't solve the problem, I'm reasonably new to this myself (but I like to learn).
I believe the primary-db needs to be updated at least once before the -C option will work. From what I have read it can be unreliable using -C because the cache should always be up to date. :\ not ideal.
After reading the above maybe it might be a good idea to have a look and see what repos yum is using. There maybe closer repos to you that will download faster that the default. There is some good advice in the above post like removing 'yum-fastestmirror' plugin to help speed things up. There is also a recommendation to use yumex instead.
Hope the above information is helpful.