Hopefully a simple Yes/No question. Whilst looking at this question on GIS SE one solution was to use IN_MEMORY. My comment was to simply add a spatial index to the shapefile. So it got me wondering does a FeatureClass in IN_MEMORY have a spatial index? I know they don't support attribute indices (I added an idea 7 years ago about that!) but was unsure about a spatial layer. Even thought IN_MEMORY is a fast place to read/write from I wondered if there was a scenario where writing to disk would be better as the dataset could support a spatial index which a tool would make use of.
Reading the help I know IN_MEMORY does not support complex datasets like networks but a spatial index seems to be such a fundamental component of spatial dataset that I wondered if it was there just not documented?