The popups that display the PlantUML logo are disruptive because they grab window / keyboard focus. Running a long sequence of individual  rendering commands is therefore constantly disrupted, and the user cannot continue other work with the rendering going on in the background. I suppression flag would be pleasant to resolve that.
 The reason for not rendering all at once is that PlantUML cannot render multiple formats at the same time, as in -tpng -tsvg which would probably be optimal from a rendering time savings perspective. So I ended up doing things like https://github.com/arpa2/identityhub-design/blob/9a8e88fb0d942539fa07e506a525e3bd23851d66/Makefile
IMHO, a commandline utility should never grab the GUI anyway... but that is just an opinion. In general, PlantUML is much more pleasant to work with than any of the graphical tools I tried! It's just this disruption of ongoing work that is, well, disrupting the pleasure of using it.