As others have said I don’t find dd very helpful when trying to benchmark in-fact I might go as far to say it can be misleading. FIO is a much better tool however even that can be misleading if you don’t understand the parameters and are unable to match them with the parameters of your environment.
I would suggest as a direct comparison to your dd test you try the below command on the pool and compare the results.
fio --randrepeat=1 --ioengine=posixaio --direct=1 --name=test --bs=1M --size=20G --rw=randwrite --ramp_time=4 --numjobs=2
The more you increase the number of job (numjobs) the faster it will perform till it hits its peak.