You can activate Windows later on. There will be no performance difference even if you dont... https://www.howtogeek.com/244678/you-dont-need-a-product-key-to-install-and-use-windows-10/
kacper03.tylko, are you interested in this unit for gaming? Most builds these days include a more robust build especially with drives. Most use an SSD for OS and frequently used applications(focuses on speed) and a mechanical drive for storage. Something to keep in mind.