There were a lot of JS heavy web applications prior to Google Maps. From around the release of IE5 at the end of 2000 in particular through the long tenure of IE6. Having worked on some JS heavy applications around that time. It was also much harder as you had a relatively wide variety of browsers and versions. Since people on dialup were far less likely to update their browsers regularly (or at all beyond what came on an ISP or AOL CD.
Of course, the efforts for larger dev teams, optimizations and bundling were far less popular before then. Can't tell you how many poorly written sites/apps carried who knows how many versions/copies of JQuery for example. It was really bad for a while.
Now, at least there are more paying some attention to it. There's still some relatively large bundles that are easy to get overloaded. I mean as soon as you load any charting or graphing library it blows out everything else. Of course this is offset between bandwidth and compute advancements as well.
There was a popular developer site around 1998-2000 or so called 15seconds.com as that was the average point at which users would start dropping off from a load. Now that's measured at around a second or two.
