Thank you for explanations. Indeed, I have HDD (320 GB), 3GB DDR3 RAM, and Internet is < 100Mbps 
Did monitoring last night, and see that after approx. 4,5 million accounts processed - some reset happens. It all starts from beginning then:
So, despite my outdated laptop HW, what is the limiting factor here? And why reset happens? I see it is every ~2 hours, once ~4,5 million accounts been processed.
@fabrice, you said “…using a hard drive or a slow SSD (like an old SATA one) will not work…”. Why so? What is the technical reason for this?