I don't know if I should call this a bug, but I cannot finish the final level due to:
1. The level overall being a bit difficult, which results in me dying, which leads to:
2. Bringing me back to the last checkpoint, but the game starts massively lagging after approximately 2nd death, it is most likely caused by a LOOOT of particles/objects being left on the surface (which do not clear out after going back to checkpoint). It becomes unplayable then, I get like 2fps (my laptop is indeed old - i5, 4gb RAM, 1gb dedicated graphics - but the game runs totally fine and smooth apart from that).
Other than that, the game is magnificent, I enjoyed it very very much so far. Thanks Thomas!