This could be an asbestos board, in which case having at it with a
skill saw is maybe not such a good idea.
Or, it could be a cement tile backer board, in which case a utility
knife is all you need (just score it and then break it -- no need to
wear out a blade).
Or it could be other things too, I suppose.
I'd recommend either (a) figuring out what it is before trying to cut
it in any way, or at least convince yourself that it isn't asbestos
before going at it, or (b) figure out some way to remove it without
resorting to cutting/smashing/etc. After all, if it is a funky
material, or concrete, or whatever, it certainly was not made in place,
and it seems unlikely that it would have been installed during
construction. More likely it was added after basic construction, or as
a retro-fit, or something.