Thank you for your help, Zerubbabel and crashdaddy!
I just found out that I can upgrade my internal hard disks and use a second SSD instead of my HDD, and I can also upgrade my RAM. But I don't know whether it is still worth the costs, because the laptop is already almost five years old.
So using an external SSD hard disk or moving folders from C to D might be the better (and less expensive) solution for my first game.
