When using the Excel Service API, it is advisable the you always call CloseWorkbook() after you are done with it. This will allow your Excel Services to free resources in a predictable manner and could potentially boost your server performance and robustness.
That said, calling any web-service call is something that takes time. Depending on the way your server is installed, the way you access it and how much pressure it is under, this can be anywhere between 50 milliseconds to 500 milliseconds (it can also take more, but that’s only if your server is under severe stress. Since a failure on a call to CloseWorkbook() is not actionable, there is no reason for you to actually wait for it to finish so you can see if it succeeded or not. Because of that, you can always* make the call asynchronously and shave some time off the operations run time.
To make the call Async, you need to do two things:
That’s it. The method will be executed asynchronously and not cost your application time.