All Outlook object model calls run on the main thread
While writing an Outlook addin, lots of people feel that they should try to help with Outlook performance by running their addin code on a background thread. While this can help in some scenarios it can actually make things worse in others, particularly if the addin is interacting primarily with the Outlook Object Model (OOM). The OOM is run in a single threaded apartment COM server, therefore every COM call is executed on the main thread of outlook.exe.
If an addin is interacting with the OOM from another apartment (i.e. on a background thread or another process) all the calls have to be marshaled to and from the server apartment (i.e. the main thread) which causes some extra overhead. This overhead is why working on a background thread could actually make performance worse and it does not exist if the addin is executing in the same apartment.
Like most things, it is a balancing act and one needs to find the proper balance for their particular situation. Just remember the key to performance is "Measure, Measure, Measure..."