Is PoE more CPU or GPU intensive?

What the title says, basically.

I currently have i5 2400 with GTX 960.

The CPU is REALLY old, almost 7 years so I was thinking of upgrade that first.

But is the game more CPU intensive? I would like to get i5 8400.

I really struggle to keep 60 fps with my current setup even while leveling. When I open abyss crack, the fps goes down to 30 and lower...

I never had FPS drops that bad with my old GTX 770, so I would assume that your 960 isn't the limiting factor - though I'm a bit surprised that your performance is as bad as it is. Have you tried using Display Driver Uninstaller to ensure your GPU driver's completely removed, then performing a fresh install of the latest driver available for your card? How much RAM do you have?

If you're going to replace your old i5 with a new i5, I would recommend at least stretching for a K model so that you can overclock it. The price premium isn't that bad and it'd help make the upgrade a more substantive one. I wouldn't want to buy a non-K CPU with a base clock of only 2.8 GHz - and since you'll have to buy a pricey motherboard with Intel's expensive Z370 chipset, not having the option to overclock your new processor seems almost criminal.

If your CPU ever fails to turbo while playing Path of Exile you're going to see pretty weak performance. In your shoes I'd probably get a Ryzen 5 1600, a decent B350 motherboard, and overclock it. Less money, more reliable performance, much better value overall.

But it goes without saying - your money, your decision.