Slow in the sense that accessing it via SPICE is slow? I’ve always found SPICE to be slow and try to get better solutions set up in the machine, e.g. xrdp,vnc
Or, slow as in command execution is delayed, the VM is hanging, etc. If it’s this one then we’d probably need to know your hardware configuration, TrueNAS version, VM configuration, and if this is the only VM you have that is acting slowly in order to help further.
EDIT: Ah, just saw you had mentioned dragonfish – have you also checked if your swap is being hammered as mentioned here?