What "viewer"?  Something of your own design?  Some built-in class?  How  
does the background thread add data to the viewer?  Does the user interact  
with the viewer as the work is done?

You don't offer much in the way of details, but if you are simply adding  
one item at a time from the background thread to some UI component in the  
foreground, then it doesn't surprise me that the operation takes longer  
and that the UI component is still somewhat unresponsive.  That said, if  
you want feedback to the user in the way of items being added to the  
viewer as progress is made, this may be unavoidable to some extent.

You could mitigate the issue by batching items up.  Let some number of  
results accumulate before you actually add them to the viewer, and disable  
updates for the viewer while they are being added.  If having the items  
show up as they are found isn't important, then the best solution would  
probably be to just keep the list of items somewhere until the operation  
is done and then add them to the viewer all at once (or maybe even add  
them to a hidden viewer and swap that in for the displayed on once  
everything's been added).

Pete

I'd like to see *your* source code - or rather, a short but complete
program demonstrating the problem. My guess is that you're not using
BackgroundWorker properly, but it's impossible to say without seeing
some code.