I just tried to install CentOS on local machine and found that when I ran 'yum install', it will always go to specific mirror url that is so slow. I know there is some cache to tell yum to use it but I don't know where on it. Try to search in google but many problem comes up that didn't relate to what I want to fix. Until I found a link below tell me where is yum cache file located. Just run command below :
rm /var/cache/yum/timedhosts.txt
Once you removed that cache file, just run yum again to see if you got better mirrors or not. If not, you may try to follow link below to do additional steps to fix it. I, myself, tried the one above and it works fine for me.
Source : ServerCommand.org