Having been working on the graphics of a multi-platform engine, do you think supporting Windows is difficult enough that when a AAA game has horrible performance on day one it is not completely unjustified?
I don't think it's so much of "supporting windows", but more towards "supporting wide range of hardware/software configs".
For big AAA games, usually (not always) most of the revenue comes from consoles. So it's natural that this is where most of the effort goes into, and most of the optimizations, and most of quality assurance.
Now, pretty much everyone also has the game running on the PC in some state all the time - after all, all the development tools are on PC, and so on. These days, with both PC & consoles even having very similar hardware (no "exotic hardware" like Cell/360) that's even easier. However, a PC has a ton of things that you don't have to worry about on consoles - various numbers of CPU cores and speeds (with unknown amount of that taken by other applications & background processes), all the different GPUs out there and their various driver versions, unknown amount & speed of memory and storage, etc. etc.
Getting most of that working acceptably is usually not rocket surgery, but requires quite some QA and then some amount of development work to fix or work around the problems that are uncovered. Game development timelines often do not leave "extra time at the end" for PC optimization -- up until humanly possible, the teams usually try to make the best game they can on the main platforms (this being consoles), and then they ship. And then once that is done & shipped, they turn into "oh we should do some QA & fixes for PC" I think -- that's just a natural course of things with consoles being the major money bringers.