Performance is great IMO, I store all my Plex media on this setup as a network share and never have any issues or slowdowns. I only use the setup as a strict NAS nothing else.
I started with 9 drives at 12tb first, about 3 years later (mid this year) i added the second vdev to my main pool. 9 drives 20tb each.
V-devs do not require to be the same size between v-devs, but they do require to have the same amount of drives in each.
I have unraid and proxmox setups on other machines running independently. Plex and other software for example all access my TrueNAS over the network.
For the TrueNAS system IMO you don’t need much “horsepower”. I run it on a 12 year old motherboard, 12gb ram and a 60gb SSD to boot. Nothing special at all. Unraid and proxmox on the other hand is where I spend the money on ram and processing power.
My Network is gigabit and I get full speed on network transfers, looking to do 10gb in the future, but that would require 10gb NIC’s in all my PC’s and new network switches. Don’t see it effecting my TrueNAS sytem setup. Besides your network transfer is only as fast as the read/write of the drives.