Many web designers have very powerful PCs and the highest possible speed Internet connections. After all, we all have a bit of geek in us and really love using the Ferrari's of the PC world. We shouldn't however forget that the site experience we get on our systems isn't the same as most users will have. If anything all that power let's us be less efficient in our site development without being able to visually detect it.
In the past PCs got a little faster and more capable each year. In some ways they were barely keeping up with software thirst for more power. The rate of improvement will continue but the PCs on our desktops are now much more powerful than most applications require. We now have new classes of CPUs that are less powerful, less expensive, use less power and still meet our basic needs. Netbooks, many based on Intel's Atom CPU, have been paired with 10" screens and established a much lower price point than their X86 brethren. Now PC use is available to those who couldn't before afford it and for new applications. At the same time the more power efficient ARM processor found in many smart phones has gotten powerful enough to run many basic applications and many of these devices have small QWERTY keyboards. Slower than ATOM but even more power efficient.
Regardless of display characteristics let's not forget users may be surfing inside a window on their much larger desktop. Particularly with the wide displays becoming common users can multiple windows open and simultaneously viewable. I sometimes watch TV on my screen in one window while writing in another. Bigger screens don't have to mean bigger browser windows to surf in like in the days 640x480 12" monitors. Some users don't multi-task and will use their entire wide screen to surf in. Some designs don't view well 1440 or more wide. Another thing designers need to deal with.
I'm using a Nokia N810 to write this article. It's ARM powered with a 4" screen with 800x480 resolution. This baby will run all day on a cell phone size battery. The user experience with these devices differs when accessing the Internet. This is of great importance to web site designers. Our target PCs now vary greatly in power and display real estate. To ensure a good user experience we need to rethink our design approaches. We need to verify our web sites on different hardware platforms to ensure the best user experience regardless of device they are using. I prefer not to do my development on on some all powerful gaming machine because it isn't representative of the majority of site visitors. My Nokia N810 is one of my test targets.
Designing with static layouts leaves mobile users constantly scrolling -- very annoying for them. Designs now need to be dynamic and flow inside a large number of screen and window sizes and aspect ratios. Web page performance is a real issue and fancy Flash pages with lots of animation are too slow on some devices. The new iPad for example doesn't even have Flash. The users of our sites have changed and we must change with them or they'll surf elsewhere. Pages that download fast are the ticket. Smaller less dense image files are required as is the need to download fewer site elements. Backgrounds load faster if they are a repeating texture rather than a screen sized image. Combine multiple images into one when possible. This is even more important for surfers that have slower Internet connections.
Your can get reasonable picture of your site visitors equipment and software choices with a tool like Google Analytics. Information is available on display characteristics, browser and OS. I'd also like to see what CPU was used to get a sense PC horsepower used by our visitors. You can also get data on connections types, e.g. cable, DSL and dial-up. This information gives me a good sense of what my surfers use so that I can design for a great experience for the largest number of users.