The quick response isn't a reflection of how easy it is to change a page. It's a reflection of the millions of dollars government agencies have spent preparing for every potential shutdown over the past few years.
Long before the shutdown happened, government agencies stopped doing the people's work and instead started to prepare for a lapse in funding. Government inefficiency caused by the same people who are tasked with oversight to prevent wasteful spending.
The web sites were able to be turned on quickly, in part, because it started happening *before* the president signed HR 2775. OPM ordered government employees back to work the day before it was signed.