Use the raw materials calculation to determine weight of each item. Also might be good to add some sort of research to reduce the weight of inventory items by a percentage(lightweight, shrink ray tech, etc...); and/or a research to increase the carry limit of a character(steroids tech, bio enhancement tech, etc...).

Thanks for the suggestions. The purpose of this mod is not realism but to create a tangible benefit to keeping a clear inventory, to help break my habit of always going everywhere with my inventory nearly full (like how the purpose of my mod Diffuse Resources was to break my habit of never building on top of ore patches); this is why slowdown is calculated based on proportion of inventory filled, instead of the absolute amount you are carrying. Adding a research to ameliorate the inventory weight would work against the mod's purpose.

I believe there are other mods which increase the player inventory size, which would help relieve the annoyance added by this mod.