The perf hit with Filevault became practically zero when Intel added the AES encryption hardware into its chips, which was quite a while ago (and definitely long before the T2 was a thing). I don't remember exactly when that was, but I remember noticing a considerable difference, because I've used FDE since it became available in Filevault. It wasn't even on Macs only, my Windows machines also showed the same improvements using (IIRC) TrueCrypt back in the days.

The dedicated AES hardware extensions in ARM and x86 cores are probably the same logic anyway, so it shouldn't matter too much who decrypts the data. Maybe it's a tiny bit faster with the T2 though, because then the CPU doesn't necessarily have to pipe all the data through it's buses for decryption then. But that is more or less a feature of having a dedicated separate chip for it, and is thus not tied to the question of whether that chip uses ARM or x86 or any other ISA.