I have disabled fast ParticleIO for now. There's a bug in this code, which I am struggling to figure out the right way to fix. This only applies to Enzo data, and the bug only showed up when you had a derived field depending on only one field, and only if it's more than a simple conversion factor. The only field I found that this applied to was ParticleAge.
During the reading, we pass in conversion factors for fields, so that when returned they will be correctly scaled. However, I was incorrectly setting dependency fields as the main field; this means that, for instance, the ParticleAge conversion was being applied to the creation_time field, and then creation_time was set as ParticleAge. This is inside yt/data_objects/particle_io.py.
The problem here is getting all three things playing nicely:
I have to sit back and think about ParticleIO. Really I have always kind of hated my particle IO implementation, but I think the solution is to simplify it and make it behave more straightforwardly. I'll notify the list when I've fixed the bug and re-enabled ParticleIO.