The current lack of a progress display is frustrating, yes, as are these prompts if you have a lot of plugins and it simply takes a long time. Cubase does show the progress as it is scanning as a line on the logo screen - not in the form of a thermometer or anything, but with the plugin name. The scanner actually should keep scanning even if you don’t respond to the popup window (because Dorico doesn’t really have control over the scanner at this point), although you might have to click “keep scanning” once when it is at the end of the scanning progress if Dorico doesn’t realize that the scan is finished.
The reason the popup window comes up is that the audio engine (which is actually the Cubase audio engine) launches as a separate process (separate program) from Dorico itself. Dorico only establishes communication with the audio engine once it has fully started and initialized all plugins - until that point, Dorico is blind to what the audio engine is doing. It doesn’t know if it is still scanning or not. The popup is displayed by Dorico which is wanting to know if you want to forcibly kill the audio engine because it doesn’t know what status the audio engine is in because it hasn’t finished initializing all plugins yet and so it can’t communicate to it yet to see what status it is in. So they put this on a simple timer, with the delay set to how long it would take for the average notation program user with a moderate plugin load to scan plugins.
Because the audio engine is running as a separate process and Dorico doesn’t know the status, it can’t currently also show the plugin names as it scans them like Cubase does, and any popup windows displayed by the plugin often come up behind the Dorico window because Dorico is in the foreground and the audio engine is in the background and it is the audio engine displaying the plugin popup.
What they said they were working on is better status reporting for the audio engine back to Dorico, hopefully they can make it so that Dorico can actually get updates from the audio engine as it is in the process of starting and scanning so it can show what it is doing. If Dorico has better information that the scanning process is actually moving forward, perhaps they can also suppress the prompts every 30 seconds.