If there is a user.config file it will use the connection string from there in preference to the one in web.config.

However there is no fallback, if it can't connect using the one in user.config, it won't then try the one in web.config.

I have never heard of any performance issues relating to using a user.config file

If you are not using a user.config file you can remove the file=user.config but I really don't think that will have any impact at all.

If it is not efficiently resolving the host name mssql07.1und1.de that could cause performance  issues

Hope it helps,

Joe