I found the problem. One of my SSDs is bad.
For those running into similar problems:
I spent some time looking at the netdata dashboard:
Here I noticed one of my CPU cores was running at 100% iowait once I started writing data to the pool. Scrolling down further to individual drives I found one drive standing out:
After physically unplugging this SSD write speeds immediately returned to normal.
I’ve tried swapping which ports the SSDs were connected to but the problem persist so long as this individual SSD is connected. Guess I’ll be filing an RMA.