I have experienced my assemblies to become slow when the number of parts, mates and references increase.

I used the Assembly Visualization tool and found a part "Part A" that had a high number of triangles (50.000 or so).

I opened the Part A and changed its Image Quality to the lowest setting and then saved the part.

Then by opening the assembly, rebuilding it and using the Assembly Visualization tool again, I could see a decrease in the amount of triangles (to about 16.000).

"Great!" I thought and did the same thing to another part "Part B", also resulting in a decrease of triangles. But somehow the image quality of Part A now returned to its previous value... ...and eventually at some moment Part B also returned to its previous value.

Worth noticing is that Part A is inserted into the top-assembly and Part B is inserted into a subassembly

What sorcery is going on here?

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Related question: Any other tips on how to make the assembly faster in terms of executing commands, rebuilds etc.?