Network browsing very slow

In my enterprise I have a Windows Server 2003 server with active directory. I
use the server has file server too. I have PCs in the same server network and
I have some PCs in another location. These PCs are connected to the server
network by a VPN created between 2 firewall Linux.

In this way I can browse file server directory from PCs connected by VPN,
but I have a problem: the browsing is very very slow.

The browsing is very slow but if, for the first time, I write the directory
path by hand in the path bar, then the browsing accelerate and I can browse
file server directory very well.

The question is: why? Why the browsing accelerate after I change the
directory path by hand? I can solve this problem without change the path with
hand?


Thank you very much for your help.