Gurren Lagann didn't have a particularly slow start... Nor did this one, for that matter. This seems to belong to the risky class of stories where the end is first laid before your eyes and then you start to rewind it backwards, figuratively speaking, as we don't have even flashbacks that much. Great many amnesia stories in general aren't like this, because the story doesn't really seem to move forward much, so the emphasis lies fully in the past and the amnesia is not just a character trait. It's more or less a dead world so it makes sense there's not much forward movement, but I still like how this isn't any flashback fest. I never liked those very much. But I like how this show proceeds with Casshern meeting random denizens and learns something of himself either directly or indirectly.