When it hits that threshold it becomes immediately noticeable - I/O performance will basically fall right down. At my old job we noticed it around 93% on Solaris 11.x, and maybe 85% on Solaris 10 off the top of my head, on two different production systems. Best practice was considered to keep pools on any ZFS system (OpenZFS or Solaris) below 80%.
If you have say a couple of 4TB disks in a mirror vdev, get it say 98% full, then expand adding new vdev with two 8TB disks mirrored, then you still have to pay the price whenever ZFS allocates from the first vdev.
ZFS tries to spread the load between vdevs in a weighted manner, which means most of the writes would go to the new vdev. But if it allocates from the first vdev you'll pay a latency price.
This means the latency in this case can be quite unpredictable, and not something that'll go away until enough blocks are freed from the first vdev.