This issue has been resolved in MSG Viewer for Outlook v3.83+.

Thank you for your patience! Please update to the latest version via the Help menu or App Store.

In some cases, users on OS X 10.11.4+ are experiencing extremely slow conversions (1+ min).

We have traced this issue to a glaring bug with Gatekeeper, which Apple introduced as part of the OS X 10.11.4 update and it remains unresolved in OS X 10.11.5. Other software developers are reporting similar issues. Since this is a bug in the core operating system, there is no known fix at this time, and Apple has provided no information on when a fix may be available. We have exchanged a few emails with Apple's software support team and will continue to follow up on this issue.