External hard drive slows down computer and makes processes freeze
This is a common issue with mechanical HDDs (non-SSD).

macOS sends a command to the external device (your HDD) but the device responds slowly. Some HDDs need to spin up in order to respond to the OS's request. Until the device is able to respond the OS may freeze.

To improve your system's performance try these steps:


prevent HDD sleep. Spinning the HDD down may save some energy but it slows down your system as well.
Disable Spotlight for the external drive. System Preferences > Spotlight > Privacy > "+". Select your external HDD and press "Choose".