Thank you for your fast response.
So I realized that this must be a weird memory leak issue that I still don't understand. You can see it in the video, that after I modify a variable that is not even referenced, the FPS decreases significantly. When I put the repeat loops in comment, the FPS goes back to the normal value, but I don't understand why, since the loops did not decrease the FPS until I modified something.
Check out this short clip:
And this is the code: https://pastebin.com/cH623DuXAs you can see, in the debugger there is a spike in memory usage and that is when the FPS starts to drop. Before that, the memory usage stays at a constant level. Does the GMS VM have a memory limit or something? Do I need to increase the memory limit somehow? How does putting the repeat loop inside a comment fix this problem?
I tested it with the old inventory's Draw GUI event too,( which is way more complicated with more loops) and the results were the same.