Game Maker's collisions default to rectangles that are pretty simple maths checks, and they're completely unaffected by sprite size, so this shouldn't have any noticeable effect on performance.
4 years ago (+1)
It would need a bit of work, but it doesn't seem impossible. player_inertia_x already sort of supports 1-pixel slopes (to solve edge cases where you could get stuck in jump-through platforms, if I recall correctly). If you replaced this hardcoded code with a loop that checks a range of values, it'd allow for slope support. (My preferred approach is to start the loop at y+SLOPESIZE when you're on ground, y+0 when you're in mid-air, and in both cases sequentially check values until y-SLOPESIZE; this lets you smoothly follow slopes down, but you won't get sucked down at massive speeds if you try moving left/right in the middle of a jump because the game thinks you're on an invisible slope)
Don't forget that you will also need to change the rectangle_free script so that it uses precise collisions instead of bounding box only, and give all slope objects a sprite with a precise collision mask.
As an example for how the improved inertia function would look like, here's the corresponding code from my SoulsVania engine:
4 years ago(+1)
Alright, thanks again for your help! Definitely gonna buy this now, if only because your customer service is so on-point.