Laptops running slow when connected to another network

We have the situation where engineers in our company go out to different
sites and connect their laptops to customer LAN's (for programming PLC's &
internet access). All the laptops are joined to our Windows 2003 domain and
are WinXP SP2.

When out on site & connected to other LAN's the laptops are very slow to
login, open windows explorer etc. If the machines are started without being
connected to the network, everything runs fine.

I guess the laptops are looking for our domain controller. I have tried
making different hardware profiles and disabling a number of services but
with the same result.

Creating a local user on each Laptop isn't an option, as they don't want to
have different profiles.

Are there any specific timeout settings to help with this issue?

Thanks in Advance

Mark