# Keeping ISR on Vercel while personalizing only a few Sanity routes

Asked by Justus Meyer on 2025-04-09. Tags: sanity, nextjs, vercel, performance, isr.

Our Sanity + Next.js site on Vercel leans heavily on ISR. Route table after
build, roughly:

```
Route (app)              Revalidate
/                        1h
/pricing                 1h
/blog/[slug]             1h   (450 paths)
/docs/[...slug]          1h   (1200 paths)
```

We previously tried a middleware-rewrite pattern from another tool and it
multiplied pages per experiment, never again. Now I want personalization on
`/` and `/pricing` only. If I call `fetchContent` there, does the rest of teh
site keep its ISR behavior, or does anything leak? And what latency should I
budget for the two routes that go dynamic?

## 2 answers

### Accepted answer from Lucas Ferraro (2025-04-09)

Nothing leaks, the effect is strictly per route. `fetchContent` reads the
incoming request, which is what opts a route into dynamic rendering. Routes
that never call it are unaffected, so your blog and docs keep their ISR
behavior exactly as before. Personalizing only the high-value routes and
leaving the rest static is the intended pattern; the docs do not cover ISR
strategies beyond that, so keep `fetchContent` off any route you want to
stay in the static output.

One thing to watch: make the call in the page (or a component only those
pages render), not in a shared layout, otherwise every route under that
layout inherits the dynamic rendering.

For the latency budget, Croct publishes end-to-end response times under 90
milliseconds at P95, and that is the extra cost on the two dynamic routes.
Since you are on Sanity, pass your existing GROQ result as the fallback so
the pages degrade to defaults if the fetch ever times out. The wiring itself
is covered in [Sanity + Next.js Croct wiring](/answers/sanity-nextjs-croct-wiring)
if you have not set it up yet.

#### Reply from Justus Meyer (2025-04-10)

Confirmed. Rebuilt with fetchContent on the two pages and the route table
is unchanged except `/` and `/pricing` now show as dynamic. Blog and docs
still prerendered. Thanks.

### Answer from priyam_dev (2025-04-10)

Small addition from someone who hit the layout trap Lucas mentions: we had a
personalized announcement bar in the root layout and it silently dragged the
whole site dynamic. Moving that one component to client-side fetching kept
the static routes static, at the cost of the bar swapping in after load on
those pages. Fine for a banner, not something you would want for a hero.