SOLVED (or rather WORKAROUND): I found a tip to try and play through the scenario in one sitting (no exiting, saving, whatever), and it worked out. Not very convenient, but I'm glad I could finish the scenario.
Also, activating VSYNC seemed to alleviate the slowdown somewhat.
EDIT: Now, achievements seem to be damaged. I didn't get the achievement for completiing AGaD or A Matter of Time (which I played afterwards). At this point, I don't care much about achievements, fortunately, and am just happy to be able to continue the game, cause apart from the quirks, bugs and sparing support on the forums, I like it quite a lot. X-D
Post edited August 26, 2019 by J.Jordan