libpq or postgresql performance

Hello,

I'm working with an application that connects to a remote server database using "libpq" library over internet, but making a simple query is really slow even though I've done PostgreSQL Tunning and table being indexed, so I want to know:

-Why is postgresql or libpq that slow when working over internet?
-What else should I do to solve this issue in addition of postgresql tunning?
-Why if I connect to the remote server desktop (using RDP or any Remote Desktop Application) and run the application using the same internet connection, it runs really fast when making requests to postgresql; but if I run the application locally by connecting to the remote postgresql server through "libpq", it's really slow?.

Thanks in advance,

Ariel Rodriguez