Having completely independent full pipelines would, obviously, be bad for performance. A lot of the operations should, in theory, be exposure-independent, and even for some that are, in many cases it might be beneficial to just have them operating on a “rough” exposure for the majority of the pipeline.
In a non-fusion pipeline, it’s usually: curve (linear in, nonlinear out), ODT, done
For a fusion split pipeline, it would be:
split, exposure shift (in linear space), possibly exposure-dependent color adjustments (in linear, see Google’s NS paper I linked), curve (linear in, nonlinear out), ODT, fuse
Things like demosaic, white balance, sharpening, scaling, lens correction, etc would all be done prior to the split since most should be exposure-independent, and even those that are exposure-dependent might behave strangely in a fusion pipeline and are best used pre-split on a “rough approximation” exposure unless you want to sacrifice performance to do something really funky.
Edit: BTW, in Mr. Wronski’s demo, the mip slider (in the dt implementation, this is the limit on the number of levels) gives a great example of how performance optimizations can affect haloing. In the default enfuse implementation, it will (except in extreme circumstances) use as many levels as necessary to get to a single-pixel pyramid tip. In wronski’s example, and dt’s implementation, there’s a performance optimization to stop early. You can see that this performance optimization leads to halos, with the halos becoming sharper/more noticeable the earlier in the pyramid you stop.