Sorry, this is a new feature and isn't quite stable yet. The idea is to tell the user that Android is optimizing system apps in the background (e.g. after the installation of OpenGApps) and the whole system might be slow and not responsive.
Those optimizations are detected automatically during the boot. If you don't like this window, you can click on the bottom and the standard Android navigation will appear - you can simply click on the home button and this window will close.
If it appears again after the reboot, it means there apps are still being optimized.
@Artur951, there is no official support for Google Play Services, but we also didn't introduce anything new to prevent the installation. You can flash the older version with our installer application (use the button "Skip login and use a local image" instead of logging in).