Main thread doesn't respond while NSOperationQueue is running its task

I use NSOperationQueue to create UIImages in the background,

upon image creation, main thread gets notified and set the image to a view.

While it works and all, main thread is not responding as I hoped while the NSOperationQueue is working.

The point of using thread 开发者_高级运维here is to let UI be responsive to user interactions.

Am I missing something here?

check Invoking thread with a single parameter