in contradiction to that, vmware letting install the hypervisor on slow sd card...
Again, the reason for this being that once booted from usb/sd card, the esxi gets loaded to the memory and from in-memory access is much faster than disk read/write. So the fact that the sd card is slower doesnt matter once you boot up
Boot a esxi image from usb disk, after booting, try to remove the usb disk, your server will function still. At some intervals, the backup.sh file will be called to write the changes during which you might face error.