A problem about VTA-FPGA generating bitstream for PYNQ-Z1

when i used ‘make’ command to generate customized VTA bitstream, I found it take about 4 or 5 hours to get done with the generation work. I knew that i could change the VTA_HW_COMP_THREADS variable according to my machine to decrease the assuming time for generating bitstream. But i don’t know what resource on my machine i need to according to. Maybe CPU cores , but i can’t figure out the magnitude relation between them, can someone give the help? Thanks a lot !