Internal Network Slower Than External

i've problem with my internal network..
here, we have about 20 server including access card server...
people complaint the access card take about 20 second to verify the card..some said the problem come from network itself..user can surf to yahoo without any problem but when using internal network, it much slower than external...do anybody here experince with this kind of problem? what is your seggestion to investigate the problem?

If this problem applies to all internal access and not just this particular server (access card server), it could be a DNS problem. It could be going out to the internet for resolution of internal addresses.

so should i check the dns server configuration?
do you suggest i unplugged the dns server and try internal connection?

I would definitely check the DNS configuration but I don't think unplugging would help.