On AOT, I seem to get 200FPS with rendering mirrors and without.
Yes, because in AoT, you have the mirror rendering: Your character, all the images it has mounted, the interior you're standing in.
In Blockland, not only would they have to be coded to work for fxdtsbricks, but also be recoded entirely because they were an interiors feature. Then there's the fact that fxdtsbricks don't derender when they're behind objects, so a single mirror in Blockland's current state could effectively double the bricklag, and shadows would probably complicate this even further.
So yeah, let's not compare 2 objects to an upwards of 100k.