When opening Office files on an iSeries share with the client being Windows 7, it can take a while before the file opens in the Office application.
A solution is to set the WebClient service to Automatic and start it. The default setting for the WebClient service on Windows 7 is Manual. On Windows XP the default setting is Automatic.
Stopping the WebClient service and setting it to Disabled seems to help even more in terms of speed, but this is not a good solution if you still want to be able to access WebDAV shares (SharePoint).
Sources:
Windows Client TechCenter > Windows 7 IT Pro Forums > Windows 7 Application Compatibility > Office 2007 and Windows 7 Slow saving file to the network post is about a Windows Server 2008 fileserver, but one of the responses describes problems with a share on AS400.
IBM - 539958206 - Windows 7 and IBM i NetServer
System iNetwork Forums - Windows 7 and Netserver Shares