When modifying custom properties from the "Edit Properties" of a node, submitting changes validates all settings on the page. This includes things like SNMP community strings, and WMI credentials. Unfortunately, when the node is "down" and not responding, this takes an extended period before it times out. You can however edit custom properties for the node without entering the "Edit Properties" view. You can edit the custom property values from within the Custom Property Editor, available under [Settings -> Manage Custom Properties -> View / Edit Values].
Altering the behavior of the "Edit Properties" view, to bypass page content validation would indeed be a feature request, and something worthy of posting to the Feature Request forum.