Incremental Static Regeneration Is a Trade, Not a Free Lunch

The pitch is straightforward: serve the cached static page instantly, then regenerate it in the background on a schedule, so visitors get speed and the content doesn't go stale forever. What the pitch leaves out is that the first visitor after the revalidation window closes is the one who can hit a slightly-stale page while the regeneration happens behind them.

For most content that's an acceptable trade — a blog post that's a few minutes out of date costs nothing. For content where staleness is actually expensive, like pricing or inventory, the same mechanism that makes ISR cheap is the reason it's the wrong tool.

The revalidation window itself is the actual design decision, and it's one a lot of teams set once and never revisit — a number chosen for build performance early on, quietly becoming the site's real freshness guarantee.