How can I retrieve Active Directory users by Common Name more quickly?

You can call UserPrincipal.FindByIdentity inside System.DirectoryServices.AccountManagement:

using System.DirectoryServices.AccountManagement;

using (var pc = new PrincipalContext(ContextType.Domain, "MyDomainName"))
{
    var user = UserPrincipal.FindByIdentity(pc, IdentityType.SamAccountName, "MyDomainName\\" + userName);
}