Spoilers:

Mark triumphally says “it’s about time.” in the first issue of Invincible when he gets his powers, and the first episode of the series is also called “It’s About Time”. I think that this is a thesis statement: that the story is literally about time.

Almost every major character has to deal with time in some way:

-Mark has to deal with long stretches of time away from his family and the consequences of that (strained relationships, missing time with his daughter, etc.) He also experiences time travel in the form of a possible universe reboot from the Time Noodles. At the end of the story, he references time: “what will you have after five hundred years?”

-Eve has to deal with his long absences as well. An alternate version of Eve spent 15 years pining for Mark.

-Nolan’s arc is probably the most central of the series. He has lived so long he’s become disconnected from humanity. Time has worn away his empathy.

-Robot/Rudy/Rex mirrors him, becoming cold and cynical over nearly a thousand years of living in the Flaxan dimension.

-The future version of the Immortal also has this problem.

-The entire Flaxan subplot is about the effects of a living in a highly altered timestream.

-Monster Girl is highly affected by time, her powers making her younger whenever she uses them.

-Lastly, in the season one finale, Debbie has a short monologue that explicitly explains what happens to Robot, Immortal and Nolan: “as we grow older, the weight of the world wears us down.”

I think all of these things are different ways of Kirkman examining time and how it affects people.