Asynchronous code completion in Visual Studio

I am developing Visual Studio extension and one of features is JavaScript code completion. I implemented ICompletionSource and other stuff base on that article: https://msdn.microsoft.com/en-us/library/ee372314.aspx. Everything works, but in my real implementation, the act of getting completions is slow (it's handled by another process). It causes that UI is frozen for a while and it has negative impact on user experience. Is there any way how to made code completion asynchronous?