I heard the price of NAND flash is supposed to crash in 2019. The problem of "network can write faster than vdevs" is ultimately one that's only truly solvable with hardware. That would mean that the only unexpected write delays would be due to disk errors (etc). Overall, it's significantly better, yes, but to truly fix it, you'd actually need to understand when seeks were going to be experienced in the write stream, and then throttle based on that.
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In such a case, the system "learns" that the pool is slow-ish, but this is a classic case of "past performance is no guarantee of future results." If you free a large amount of free space on that highly fragmented pool, the system once again learns that "it's fast" and you can cause it to be stunned again when it eventually has to relearn that hard lesson. However, there's a more insidious issue, one that's more difficult to fix: when you reach a state where the pool has a lot of fragmentation, ZFS has a hard time learning the pool characteristics based on the write stream because it really comes down to how many seeks are incurred during write operations. I leave that as an exercise for the reader.
You can pretty reliably cause it to happen on a newly restarted system. Experimentally, some variation on this is still an issue with the new write throttle. I used to talk about 1531 and write_shift value of 3 where you could have 4GB write buffer sizing on a 32GB box but this would be crazy-big for a host with only four disks (as an example) - assuming mirrors and 200MB/s write speed, that's 10seconds best case to flush 4GB. If you have a slow-ish pool and the system hasn't yet "noticed" that because you've never plateaued at max write, you still get that massive first freeze if you stun it with a flood of writes. Your "fast network + slow vdevs" is one specific example of this, but the problem is more generalized. If you "sneak up" on the current system, you can still cause it to choke if it hasn't adjusted to the general IOPS capacity of the pool.
While the old tunables aren't useful/don't exist today, the problem definitely still exists. HBA controller - Asus Pike 2108 (LSI chipset?) - configured as JBOD Inet6 fe80::1%lo0 prefixlen 64 scopeid 0x5
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