This is less of an issue today (although there aren't very many jailbreaks to take advantage of it) but back then it was pretty clear the device ran with very little RAM to spare. The best tweaks were fairly simple when it came to RAM usage but true multitasking or other things that stayed resident generally had a massive hit on system performance.
iPhone OS actually could multitask with jailbreaking, though you quickly discovered the limitations of that amount of RAM.