How can I downgrade the number of elements in a mesh so Blender won't freeze so much? + How to erase flying bits in the stl? I have the following stl. I need to fill the hollow ribs and holes in them, join the cartilage to the bone and erase all those little fragments of other stuff that don't belong to the thorax, and downgrade the number of elements. Is there a way I can make all this? Blender keeps freezing...

I've tried the clean up and smoothing, but it doesnt seem to work...I have no idea why blender won't let me erase all those bits without connection...
This is the .stl.  https://drive.google.com/file/d/15qzEGg57-wGEAqz72-DykcWi0BJSQ4ao/view?usp=sharing
you could press h to hide certain elements and then add them back in the final render