Opening the Share To UI takes a few seconds. It should be instant. I am not sure where this delay is coming from. Could it be the resources or frameworks that the Share To extension depends on? The extension does load because the delay happens between displaying the blurred InitialViewController, which shows instantly, and showing the ShareViewController.
Fixed with https://github.com/mozilla/firefox-ios/commit/1d65295d1fddb8af78cb58ca06f9ab9837e0c18d The problem here was that presentShareDialog() was not called from the main thread.