I'm working on a system now, and for the first time in my career, I'm taking the time to really do it right (mostly because, for the first time in my career, I'm being allowed to). Even though it's mostly used in-house, we do have outside customers with access so I do make sure it runs acceptably on slow connections or devices.
Images are small and scarce. That's not exactly a big deal these days, but honestly, it makes styling easier.
There is one CSS file. It caches just fine. I just ran a test simulating a 22kbps connection - the first page load is about ten seconds, but after that, nothing takes more than a second or so to load, save for one page I've already marked for revision.
There are no ads or tracking. Not even Google Analytics. The designer wants to add it but I'm not gonna let it happen. We have server logs we can analyze for all the info we need, why would we send that on to Google, not to mention the perf hit?
Tested in current versions of Firefox, Chrome, Opera, Vivaldi, and Edge, and also IE10, under Windows and Linux (for browsers that run on Linux). IE misses some styling features, Firefox has some alignment glitches I haven't gotten around to yelling at the designer to fix it yet, but everything is fully usable. Safari and OS X testing will happen if we ever get a Mac to test with. Mobile browsers technically work but the site is very information-dense and nobody in their right minds would use it on a phone. (And I've tested it with Lynx, you can read it but it's not easy, and saving stuff is broken... if I ever run out of stuff to do, I might fix that, but it's not exactly a priority)
On a normal business-class internet connection (25Mbps shared across a dozen people), and a normal business-class browser (dual-core, 8GB RAM, Windows), most pages load in about 30ms. Only one takes longer than 100ms, and that's already queued for revision.
The fundamental problem is that too few programmers have a direct profit incentive to make a good website user experience. The big sites make money from ads, which are inherently anti-user. Contractors (which I used to be) care more about getting it out under the hours estimate than about quality. I work directly for the people who use this site eight hours a day, you can bet your ass I'll make this a good site for them to use.