A new developer and QA feature available in SharePoint 2010
You can easily turn on the developer dashboard, a way for the developer to check the performance of the page, by running a simple stsadm command (yes I am still using it):
There are 3 states: on, off or ondemand and here’s the syntax:
stsadm -o setproperty -pn developer-dashboard -pv ondemand
stsadm -o setproperty -pn developer-dashboard -pv on
stsadm -o setproperty -pn developer-dashboard -pv off
Off = Always off, usually used in production.
On = Always On, for development purposes, however I would say that is annoying to see it on all the pages and not recommended for production.
OnDemand = The user has the possibility to turn it on or off as needed using the small icon to the upper right hand corner of the page; you click the icon to toggle the dashboard on and off.
Developers will be able to write performance information into this Dashboard using the new SPMonitoredScope class
For more great info about this new feature you can read this post: http://blogs.technet.com/speschka/archive/2009/10/28/using-the-developer-dashboard-in-sharepoint-2010.aspx
The OnDemand option gives you the best flexibility and is safe to be used in production during the investigation process.
[Update] If you get “Access Denied!” error on Windows 7 then just run the Command Prompt as Administrator and will succeed.