Hi! A few things to check. It is possible there are still performance related bugs in Inventor, after recent significant improvement in performance. It could be files or workflows specific.
I don't think running programs in the background would cause Inventor to slow. It is because as long as there is enough RAM, other programs should be loaded on system RAM, which should be fast. If swap memory is used, it will be slow. But, it should not be your case.
You mention "turbo" CPU. Do you overclock it? If yes, I suggest you do some testing with it off. Do you see a difference? The reason why "turbo" may not help is that it is forcing CPU to run at a speed it was not designed to do. It may appear it runs faster for certain tasks. But, the turbo may also interfere with multi-threading operations negatively. I suggest you turn the turbo off and see how it works.
The network could play a role here depending on how your files are saved. Do you access files from a shared drive (libraries)? Where are Appearance and Materials libraries stored?
You said that you work on 1.5GB assembly. I assume you are talking about the size of entire files. How much RAM is consumed when it is loaded? Try turning off Express mode (Tools -> App Options -> Assembly -> uncheck "Enable Express Mode"). This option will store assembly graphics at the top-level assembly, which makes the iam file big. It helps speed up file open and see the graphics faster. However, the need to save the such big chunk of graphics data can slow things done somethings (while saving).
Many thanks!
Johnson Shiue (johnson.shiue@autodesk.com)
Software Test Engineer