I've created a couple volumes in TrueNAS and also created an Ubuntu 20.05 VM where I've mounted these as raw disks. Testing the speed is very hit or miss so far. It starts off okay for a certain amount of data but then drops off to very very slow speeds the disks no longer really become usable in the VM. I'm seeing quite a few of these in the TrueNAS logs coming in batches of 5-6, didn't see much information on that in a quick search.
isp0: Firmware timed out on command
isp0: isp_handle_platform_ctio: CTIO7[129300] seq 0 nc 1 sts 0xb flg 0x1 sns 0 resid 0 MID
This is specific to write speed, read speed never seems to be affected. In iotop I see the IO wait at 100% while the write operations are occurring.