well, this 8 core thing is just misleading lol and why?
it works on 4+4 cores and each set work at a time!
you got 4 A15 cores for heavy tasks and 4 A7 cores for simpler (light) tasks!
what this means? the phone never uses the 8 cores at the same time! it only uses 4 cores at a time, like i said above, depending on light/heavy tasks!
so in the end, you still have a quad-core lol and you should treat it like so!
then what's different about this octa chip? well despite still being a quad-core, it sure is a high-performance one and they claim the 4 A7 to be more power efficient!
but don't be mislead to think it runs 8 cores at the same time, because it doesen't lol
and yes, quad-cores can also lag, specially with Android! specially when you have a crapload of software gimmicks, bloatware and apps!
edit: if my explanation was insufficient or i didn't make myself clear (english is not my native language) read this -
Samsung Galaxy S 4 with Exynos Octa-core: what's different?