I have to say my experience was pretty much the polar opposite of yours. I played in OpenGL and has no performance issues (given that the map is intended for OpenGL, and very much set up for it, I wouldn't even want to play it in software - I think I'd rather pass than play). I quite liked the slow start weapons-wise and felt that the shotgun arrived just when I felt it was time to get one. Yes, some of the new enemies, particularly the ones that you identified, are tough and dish out loads of damage if you get caught out by them but I actually like that kind of "realistic"* enemy and didn't find them too difficult to deal with.
*by "realistic" I mean something that has believable-ish human hitpoints and does an equally reasonably believable amount of damage with a weapon that, in reality, could kill you with one shot.