I haven't used it in a long time, but Textpad was my go-to ultra light. At the time, it only supported ASCII (no Unicode) so it only used one byte per character

I haven't used it in a long time, but Textpad was my go-to ultra light. At the time, it only supported ASCII (no Unicode) so it only used one byte per character. I frequently had to open files that would hit 32 bit memory limits.