Slow write speed with high end config

I think that falls under the following warning

Important: the pool depends on the sVDEV to function, so if the sVDEV dies, your pool dies also. Take the same care with sVDEVs regarding redundancy and resilience as you did with the other VDEVs! Do not use crummy SSDs for the sVDEV unless you don’t care about pool life expectancy..

Special VDEV (sVDEV) Planning, Sizing, and Considerations