asp.net web/app.config read/write performance - configs

appSetting.config file

<appSettings>
  <add key="MYKEY" value="4ee7541c-d6fc-4f0f-a9bb-0d10fc56aafc" />
</appSettings>

web.config (part of file)
<configuration>
...
    <appSettings configSource="appSettings.config" />
...
</configuration>