How to resolve a slow People Picker in SharePoint Foundation 2010

People Picker slowed down from instant results to around 20-30 seconds. This made searching for users a nuisance, especially during live meetings. 

In our case, I believe that the issue was due to an old domain being turned down and SharePoint still attempting to resolve it.

The steps below were performed using the STSADM.exe console which was stored on the SharePoint server hosting the web application. It can be located at:

C:\Program Files\Common Files\Microsoft Shared\Web Server Extensions\14\BIN\stsadm.exe

 Run the following command:

STSADM.exe -o setapppassword -password key

And then the command:

STSADM.exe -o setproperty -propertyname peoplepicker-searchadforests -propertyvalue "Domain:yourdomain.com" -url https://yourwebapplication.com

This should result in "Operation completed successfully" and if you test, search results should be instant again. If you have multiple domains, you can add them to the previous command using commas. Example:

 STSADM.exe -o setproperty -propertyname peoplepicker-searchadforests -propertyvalue "Domain:yourdomain.com,Domain:yourseconddomain.com" -url https://yourwebapplication.com

Hopefully this helps you!