After two weeks of research and a lot of thinking, it looks like my problem (Explorer is very slow after connecting external HDD) is caused by USB 3.0 ports.
In short: after working around one hour on a battery and then connecting power and external HDD or pen drive, CPU usage is going extremely high (at least 40-50%). With help from people around the internet, we figured out that this problem happens ONLY on USB 3.0 ports. If I will connect pen drive or external HDD to the USB 2.0 ports, everything is normal.
For me, it looks like it could be a software issue, but I'm not sure what I can do with it. Do you have any idea what might help?
My laptop is Samsung 550P5C-S04.