How to Improve XML read write performance

[boyG]

I have two .NET applications running independently(can start in any order, or may be only one running) which uses XML as data store. So both applications can read and write to XML file.To keep the data updated , i'm loading the XML file every time from the disk before read and write operations. And i'm using XPath query for querying the particular node.Now there is performance issue observed in this method as there are read and write requests on XML every second from one application(uses polling, and cannot be changed)I'm not sure what exactly is causing the performance hit, but i believe its continuous read write. I tried using memory mapped files from .NET 4.0 but i'm restricted to use .NET 3.5 and not any higher versions.Can anyone help me out on this?Note : XML nodes have some common attributes , different number of attributes and one ID which i use for XPath querying.