Sounds like some sort of COM marshalling is going on.

In single apartment threaded COM objects the calls to methods are marshalled - i.e. the methods aren't actually called directly but run through a 'seperation' mechanism.

If you start up your app in a multi threaded apartment mode and enable your com object to support 'both' single & multi threaded apartments I think you'll find that it won't slow down.

Darwen.