In my experience, for HDD, when I make it near to full, my laptop slows down. Whether the eHDD or not. So, I already get in to this habit to save a lot of space. Delete the unneeded like videos I didn't open for a year.
Old videos? 643GB lot of space i could have. Well i will trying cleaning my hard disk with something what not support or not working or older version software or double files. I could sayed i have big mixer files. One is good for emergency i have one hard disk to backup my files. If my hard disk will somehow died or crash system data files are save.