I do also find it very suspicious that your first graph had a large number of storage servers all struggling, but your second one had only two.
I don’t see anyone mentioning it here, but if you’re starting from an empty database, then FDB is going to have to compete with your workload to try and split your data sufficiently so that the writes are spread across the servers.
Our own write bandwidth tests pre-load a large volume of data to create enough shards to distribute writes well, wait for data distribution to finish, and then run the actual write bandwidth test.
KrzysFR posted an example of this with good stats from his tooling before.