Server slow - how to check if DNS is wrong

Hi,

My SL server is terribly slow and I am wondering if the cause is linked to the DNS.

Problem is I am not sure how to go about this.

Anyone have useful pointers to tests the DNS setup?

Thanks for your input

Just a few extra infos I have noticed;

My setup has an internet entry router (with a wifi called EXTERNAL - for simplicity) that is connected to a TC (with wifi INTERNAL) in bridge mode. The server is connected via wifi to the TC and has a fixed IP address. All incoming traffic is directed to the servers IP.

If I connect my iMac to the server using INTERNAL the response times are real bad.

If I connect my iMac to the server using EXTERNAL the response times are decent.