Are there rate limits or a per-second cap?

There's no per-second or per-minute rate limit today — the only limit is your monthly quota. Each live response includes RateLimit-Limit, RateLimit-Remaining, and RateLimit-Reset headers, plus a Vatnode-Quota-Warning nudge once you're near your limit, so you can track headroom from your own logs. Full header semantics and examples are in the rate limits reference.