Is it possible..

that time itself is slowing down or speeding up? Would this possible acceleration account for an extra dimension of time? Probably stupid, but I'd like to know how it would be wrong. The thing about the big bang => 98% of matter in a few minutes seems a little weird, and if time is changing, maybe that would account for it?

A variable speed of time would have NO impact on the equations that give us our timeline for the Big Bang Theory. There's nothing to "account for" except you not understanding the basic concepts at work here.

The Universe is a closed system. This means that everything is measured relative to itself. If it's time "flow" is variable, it would only matter to someone OUTSIDE of the Universe. To anyone inside that variable time "flow", Time would appear to be constant. Even if it stopped completely for a while on some sort of meta-timeline, we wouldn't even be PHYSICALLY ABLE to notice. It's like the pause button or a slow motion code in a video game: the in-game characters can't tell their timeline is being fucked with.

Now then, it's time to let you in on a secret: Time and Causality as we know them didn't even exist until AFTER the Big Bang had started. LOL WUT? Yeah. The Universe is a weird place, man. Read some books like "The Fabric of the Cosmos" for a good introduction to some of these concepts.