This tip is for Windows SharePoint Services 2.0/3.0, SharePoint Portal Server 2003 and Microsoft Office SharePoint Server 2007 and may apply to SharePoint Foundation 2010 and SharePoint Server 2010.
When entering an account to a securable SharePoint object, the “Check Names” function is very helpful in being able to just type in the account name or full name into the box and clicking the person with the check icon to resolve the account.
But did you know that when typing in an ambiguous name, like Chad Schultz, instead of an account name, like, DOMAIN\cschult may take longer to resolve the account? This a because the check names function will not know which Active Directory domain the ambiguous name is in so it will try to query every domain that has a trust with the domain that the SharePoint server is a member of to match the name to an account. If there are many trusts, or any trusts are experiencing issues, then the performance of the check names function will be negatively impacted.
There is an update for WSS 3.0 that can help mitigate this issue somewhat; http://support.microsoft.com/kb/974989, but the take away is to really make sure your Active Directory infrastructure is healthy and that all trusts are up and communicating and that typing in an ambiguous name instead of an account name has the potential of being resolved much slower.
*Note: If you are installing the above update on MOSS 2007, also install the corresponding MOSS update on each server in the SharePoint farm; http://support.microsoft.com/kb/974988.