Another metric to watch for is event loop latency, as you might not need the CPU at full load for Node.js to experience performance issues.
Once the event loop starts to lag, things slow to a crawl or get out of sync. It seems to happen in high-load scenarios due to garbage collection, a huge number of callbacks, promises, I/O, or many other potential reasons that can block the event loop.
And that perhaps combined with Async Local Storage/Async Hooks, which Meteor now heavily reliant on, might cause a bad combination in some extreme cases.
We will look into it to understand what might be happening and will keep you posted.