I wasn't very old then but the main thing was RAM. Fuckers in Microsoft sales/marketing made 1 GB the minimum requirement for OEMs to install Vista.

So guess what? Every OEM installed Vista with 1 GB of RAM and a 5200 RPM hard drive (the "standard" config for XP which is what most of those SKUs were meant to target). That hard drive would inevitably spend its short life thrashing because if you opened IE it would immediately start swapping. Even worse with OEM bloat, but even a clean Vista install would swap real bad under light web browsing.

It was utterly unusable. Like, everything would be unbearably slow and all you could do was (slowly) open task manager and say "yep, literally nothing running, all nonessential programs killed, only got two tabs open, still swapping like it's the sex party of the century".

"Fixing" those hellspawns by adding a spare DDR2 stick is a big part of how I learned to fix computer hardware. All ya had to do was chuck 30 € of RAM in there and suddenly Vista went from actually unusable to buttery smooth.

By the time the OEMs wised up to Microsoft's bullshit, Seven was around the corner so everyone thought Seven "fixed" the performance issues. It didn't, it's just that 2 GB of RAM had become the bare minimum standard by then.

EDIT: Just installed a Vista VM because I ain't got nothing better to do at 2 am apparently. Not connected to the internet, didn't install a thing, got all of 12 processes listed by task manager, and it already uses 500 MB of RAM. Aero didn't even enable as I didn't configure graphics acceleration.