Internet connection slow on Mac (Solved by editing DNS)

After I got back from Japan-California trip, the internet connection on my Macintosh (Mac) seems slow. After days of holding slow connection and hours of analyzing phenomena, I realized that only look-up wsa slow and it was not slow after the connection with web server was established. I also found it was not slow at all using ethernet cable. Then I found that a server which doesn't exist in my LAN environment was set as default DNS server in DNS setting for wireless network, and the actual DNS server was secondary. So I removed the default server, and then look-up becomes normal.

DNS setting window on Mac. In the display area on the left, there was another server which doesn't exist.

The reason the non-existing server was added was not yet clear. I indeed used different LAN setting in my home of which IP address range was the same with the one in DNS setting, but why did it remain in the setting even after I moved to my room in US? Maybe the DNS setting on Mac is not automatically updated, as it is in the default setting on Windows.