TLDR

Accept possible data loss and disable SYNC on the dataset in TrueNAS Scale.

Log in to TrueNAS Scale, navigate to Datasets, select the Dataset which is performing badly. Edit Dataset Details and disable SYNC.

Seriously; Think about if it is bad for you to lose data when power loss occurs. Because you will lose writes in that case.

My use case is reading big files remotely, thus I value performance over potential data loss.