So you've got low memory and you're running from a live CD or USB stick? Live systems use ram disks so you're actually running a system with even less available memory! The CPU, in my humble estimation, is therefore flailing about wildly trying to keep whatever application you're running with hee-haw memory to play with and hence showing heavy loading! You've already diagnosed the problem as lack of memory, the CPU's fine.
Note: hee-haw is alternative Scots for nae or b*gger-all.