Hey hopefully someone can help me out.

I'm approaching half way through the single player campaign in the original Dungeon Keeper, and now it feels like the computer is struggling to play it. Serious slowdown issues almost turning it into a slideshow.

I've not changed any Dosbox settings or anything, and I'm running full screen on a mid 2011 iMac 27inch (2560x1440) fullscreen.

Any advice?

Make sure Dosbox is getting enough memory to run the game properly, go to the dosboxDK configuration file and check the memsize. My memsize at the moment is set to 32, and that runs the game just fine, even in a big battle. You might also want to check the CPU settings and make sure you set the cycles to max, and core and cputype to auto.

I have same problem, and I increased the memory available to 128 or any number, and increase de cycles using ALT+F12, but problem persist. Actually I got notice problem of FPS is quite heavy with dragons, or when you are in first person with gosht, or dragons which have strange 1st person view.
The only solution I found is click alt+r, which makes resolution drop to 640 i belive and the performance boost.

I find DK1 chuggs too much on the higher-res, so I play it on low-res; which is fine for me since I originally had the REALLY old version of the game and was used to playing it low-res by default.