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.