Thanks for the suggestions!
The display is just the iMac built in 27" LCD display (no additional monitors connected):
Resolution: 2048 x 1152 (note I've picked this and also picked lower resolutions when choosing the window size)
Pixel Depth: 32-Bit Color (ARGB8888)
Graphics chip: NVIDIA GeForce GTX 675MX 1024 MB
I'm not using Steam - both times, I've downloaded from the Spiderweb Software website.
With no other applications running and after freshly restarting, I made it a smidge further (talked to the prince, tried to open the chest *flicker, flicker*).