If you use more than 15 GB of data in a cycle on the Fi Flexible plan or more than 22 GB in a cycle on the Fi Unlimited plan (less than 1% of individual Fi users as of Jan. 2018), you'll experience slower speeds (256 kbps) above those respective data thresholds until your next billing cycle begins.
If you need significant amounts of high speed data, you can opt to pay $10/GB for the data you use past the data threshold for your plan in a given cycle (15GB for Flexible or 22GB for Unlimited).
These data thresholds are based on individual data usage, not group data usage.
Since Google isn't actually deploying the cell networks and they actually use a mixed set of providers, I am not sure if this assumption makes sense.
If the mobile network T-Mobile is overloaded with T-Mobile customers and has a small fraction of Google ones, then Google will tell T-Mobile to throttle the customers that exceeded their contract limit?
Of course they are, why wouldn't they? It's not like it's any worse than any other "unlimited" plan you can get and giving out 10x more usage at 0x more cost isn't helping their margins while trying to get heavy users to pay per gig is.
So it will be equally fast if only a few people use the service (e. g. outside of populous areas), and much slower otherwise.
So it's not like they are going to have their network underloaded just to squeeze more money from customers.
If that hypothesis is true, it does not sound that bad.