What part of the PC related to the framerate drop of huge amount of parts ?

[FirstSecondThird]

Actually, the huge lag spikes when you have a high part count are entirely CPU related and previous to 1.1; RAM related. It's because your CPU is so stressed calculating the physics for so many parts that it needs to slow down the framerate in order to let the CPU rest a little bit more between calculations. But, a solution that mitigates a small amount of the problem is to lower your Physics delta time per frame to around 0.04 or 0.03. Doing this will slow down the game time to allow the CPU to relax a little bit on the calculations without sacrificing framerate. The problem with this being, sometimes launches can take ages if you use high part counts. So, new approach that solves the rest of the problem. Use a part welding mod to weld a section of parts in your rockets or planes into a single part to lower part count. It allows you to have faster physics without sacrificing the useful parts of your vessels.