If you're running Visual Studio 2010 on XP or Vista you may benefit from installing this upgrade: http://support.microsoft.com/kb/981741

Applications that use Windows Automation APIs can significantly decrease Microsoft Visual Studio IntelliSense performance if Windows Automation API 3.0 is not installed. For example, the Windows pen and touch services can significantly decrease Visual Studio IntelliSense performance if Windows Automation API 3.0 is not installed. This article describes how to install the Windows Automation API 3.0 update. This update is available as a stand-alone download for 32-bit editions of Windows XP and for Windows Server 2003. This update is not available for 64-bit editions of Windows XP. The Windows Automation API is a component of the platform update for Windows Vista and of the platform update for Windows Server 2008.

Note that if you're running Windows XP SP2 you'll be told the patch isn't applicable, that's because you need to upgrade to XP SP3 first.  If you're running Vista or Windows Server 2008 you may already have the upgrade, since it's part of the automatic updates.  If not you can download the upgrade at the link above.  Windows 7 and Windows Server 2008 R2 ships with with Windows Automation API 3.0, so no upgrade is required.

The problem is that earlier versions of the Windows Automation API try to read the entire contents of list boxes that we post to the screen, which blocks us from virualizing them.  This is particularly a problem with intellisense as the number of items in an intellisense list is HUGE and it pops up on every character you type.

The API is typically activated when using accessibility devices (such as screen readers), pen / tablet computers, or touch devices, but some software activates it anyway (including iPhone synchronization).  Once activated it affects the entire system.