Invincible fans forget the show was always slow paced
Season 1 was incredibly slow when you realize most of it was just world building and character development. The animation distracted people from the pacing.
Season 2 was also world building and character development, with a little less stellar animation. All of a sudden, fans didn't like how slow it was, forgetting that most of it was laying the ground work for the direction of the story moving forward.
And believe it or not season 3 is also that and more. The show always looks like it's picking up more at the last two episodes.
I'm not sure what this new generation of viewers being incapable of enjoying a slow burn, especially when you consider there was a time where TV shows use to run for 20 episode seasons and half of those episodes were technically filler. Hell the flash was pretty much that and more. Which is an actual criticism of the story.
Invincible isn't long enough to be considered filler, especially with what we've seen definitely will be making a comeback in later episodes down the line. Filler means none of it gets brought back up and none of it had anything to do with anything.
It was never just the Viltrumite war. It's everything that lead up to that. The show is based around Mark's life