The laptop specifications are:
- Lenovo z580 3.0usb
Would it give any foreseen performance problems?
depending on your available bios options You can set up the external drive with its own grub bootloader, then that bootable drive will not care what you do on other drives
I read the "requirements" you are talking about myself and IMHO, they are incorrect.
Although I haven't built the original Android kernel, I built the one of Sony Xperia ZR both on a PC booting Ubuntu from external HDD which has only 2GB RAM and 4GB swap, as well as on a VirtualBox with a similar spec. Could be that the given requirement" are only for people who want to do parallel builds (I read there is a Makefile option to run many jobs in parallel, which would probably cause a multiplication of resource consumption) which I didn't use.
During the build (which took around half an hour), I had an eye on the System Monitor and to be honest, I was surprised how little RAM the build used.
Then again, I read that the original AOSP compiles PC software as part of the kernel build as well (which I don't think the Sony build does), so that could make a big difference. But I'd definitely give it a try!