Is Nasus Curator of Sands ability slow speed?

No — Nasus's ability isn't a speed at all; it's a triggered ability. Curator of the Sands reads "When you play a unit, gear, or activated ability with Energy cost [7] or more, you may exhaust me to ready up to 2 runes." Because it starts with "When," it automatically fires and goes on the Chain whenever the condition is met — no Main Phase or Open State required, and it can trigger mid-chain or even on your opponent's turn (if you play their cards, etc.).

The "may exhaust me" is the cost paid as the trigger resolves, not an activation. So it never needs priority to "use" — the trigger itself creates the window. Contrast that with his activated-ability counterparts, which would need the [Action]/[Reaction] keyword to move outside a Main Phase Open State.