The reason for this question is that many times clients, friends and parents ask me why their perfectly working old PC cannot be used online anymore. I am looking for a good answer to those people and myself (mostly myself).
In 2000 I worked on a Pentium2 400MHz PC and I remember surfing graphically rich sites, watching RealPlayer embedded videos and enjoying Flash movies.
Yesterday I fired up that old pc and went online - the PC slowed down to a crawl on most sites I tried to open.
The sites I was viewing just had images, text and one or two flash banners - just like most sites in 2000.
Here are my theories - correct me where I'm wrong:
Heavy JS usage. In the 90's you clicked on a link and that took you to another URL. Now clicking you get modal windows, sliders, social voting, etc. JS is known to be heavy on the CPU, even if the browser does some on-the-fly compiling it sill has to be done by the CPU.
The way browsers are made Browser engines are getting more reusable. Well done abstraction layers allow the same engine to be used on PC's and devices. Abstraction takes a toll on the CPU, because the engine is not made specifically for that hardware architecture.
Flash, HTML5 video Many Flash banners have rich animations and many times they are ineffectively made. Flash video and HTML5 video nowadays requires you to have a modern GPU.
Modern CPU architectures Today I work on a 3GHz CPU. Technically that should be around 6x times faster than my P2 400MHz. Actually it is even faster because modern CPU's do not work the same as my P2. We cannot compare the MHz values anymore because of multiple cores and other new technologies. That is the reason we are now talking about architectures (like Haswell) rather than GHz values. So, since the CPU so much faster it can easily handle jQuery, Google maps and flash video on the same page.