I'm playing partially on M1 Max and I hadn't played for a bit bit since the patch came out on Windows because I need cross save with Steam Deck to work. It does seem noticeably slower now than it had been. I'd just put it down to having just arrived in Act 3 as I remember complaints about Act 3 performance at release. I'm not normally super fussed about frame rate and will push the settings higher than they probably should be to get it looking prettier rather than smoother. But it's at a point where it's testing even my patience now, and turning the settings down isn't seeming to make much difference. I'm playing on Ultra at the moment. Only on turning all the way down to Low and 1080p does it feel like it's maybe getting performance similar to what I've had all through the rest of the game.
CPU-bound?. That's rare as I said I didn't noticed any issue, tested loading multiple locations including Act 3 and runs as always. But I use a M1 and the issue seems to affect Pro and Max chips, which differs their CPU cores configuration.
Try to open the system Activity Monitor and show the CPU history (Cmd + 3), then play some time and look if there are cores at 100% while others are lower, as the 100% ones would be the bottleneck.
