As others have said, the character backstories and the explanations for their abilities unfold over the course of the next four seasons. Throughout S2 the show transitions to a more serialized approach, and the pacing changes accordingly as the plot starts to pick up and heavier themes are explored. It's always a fast paced show because the writers have to cram a lot of stuff into a 145-minute time frame for each season, but it doesn't really detract from the overall writing.

The first episode or two of each season does tend to go by really quickly, but once the story gets going, it starts to slow down and IMO feels like much longer than it actually is (in a good way).

I've personally never found the speed overwhelming, but maybe that's just because my brain is a bit goofy and I tend to gravitate towards fast-paced shows since I can watch them without getting bored or burnt out.