Indeed, a Core 2 still has its uses - my primary laptop was still a Core 2 Duo until 2 or 3 months ago. But popular ad laden sites used to be so desperately slow as it went off to all the various ad agencies....
With regards to modern software, the issue is 2 fold - modern millennial devs use all these wanky, inefficient languages like Go to write stuff in, which are slow, and their natural coding style is for development speed, not runtime, so you have all this bloat of massive frameworks to maintain in memory. On the upside, its unlikely software in the future will be written in Java, due to Oracle's view on extracting as much cash from everyone.
An old DOS or 16bit Windows program had little choice but to be written in C/C++ until the likes of VB and Delphi came along. So will always run well (for those you can run through an emulator or virtualise) on modern kit, and is boosted by the improvements on modern gear. Try something like Windows 1 or Windows 2 in a virtual machine on a modern machine, and it runs spectacularly well.
My PC in the garage - and oldish 3rd gen i5 - runs windows 10 as a general purpose OS so I can look up stuff on the internet, but also runs an XP VM for all the car diags stuff that runs better under XP. That virtualised XP runs better than XP ever did back in the XP era.