I am constantly bothered by the Generating PDF dialog box for two reasons:
First, it doesn't play well with my dual-screen setup. If I have InDesign on one monitor, the dialog box often shows up on the othe rmonitor, blocking whatever applications or windows I am working on outside of InDesign. Then I must manually drag it over to the InDesign monitor to get it out of the way. Come on Adobe! Many users have multiple monitors. Your apps should remember the locations of all dialog boxes, windows, and panels and not arbitrarily show up in random places.
Second, and more importantly, there is NO reason for this dialog box to appear at all.
If I select multiple files in an InDesign book panel, then choose Export Book to PDF, the dialog box appears for the duration of the export.
But if I simply open a file and chose File > Export, the export happens as a background task as it should, allowing me to continue working on other files in InDesign.
Why isn't background exporting the default? At the very least, please make it an Adobe PDF Preset option? 
BTW, I'm using the latest version of InDesign (18.5), but this problem has always been present in all versions over the past decade.