Re: New computer decision
Even a 10 year old XP computer can watch YouTube videos without lag. I believe your problem is more software based. Do a fresh install, and watch your PC become faster.
As for the all in one PC's, it's all preference. They are alright, but they are not expandable and is less upgradeable than a desktop.
Even the monitor can't be upgraded because it's a part of the computer. So when you buy anew PC, you also need to buy a new monitor. They are also not that popular, so manufactures don't make that many models. that makes it harder to find exactly the one that fits your needs at a good price.
However they look good, don't require as much space and you are eliminating some cable mess.
So if that's what you want, go for it.
MOBO: Asus Maximus 6 Hero - GPU: Gigabyte GTX 980ti xtreme - CPU: I7-4770K - PSU: Corsair AX760
RAM: Corsair Vantage 2x4GB - SSD: Samsung 840 Evo 512GB - Case: NZXT H440