fix: useFetch perf is significantly worse than useAsync

[manniL]

🐛 The bug
When using useFetch, TTFB is ~250-300ms on an almost blank page.
Replacing the call with useAsync yields a significantly lower TTFB of ~30ms

🛠️ To reproduce
Steps to reproduce the behavior:

Go to https://github.com/manniL/nuxt-capi-usefetch-overhead and follow the README:

1. Install deps via yarn
2. Launch the application in dev mode via yarn dev
3. Go to http://localhost:3000/async and http://localhost:3000/fetch in new tabs
4. See that the TTFB is roughly a magnitude different

To ensure it is not caused by underlying fetch:

1. Go to Go to http://localhost:3000/async-o and http://localhost:3000/fetch-o in new tabs. These are options-api "equivalents"
2. See that TTFB is roughly the same

The same scenario can be observed in production as well.

🌈 Expected behaviour
Having both helpers yielding roughly the same TTFB given the same use case

UPDATE: The Nuxt 3 useAsyncData implementation does not have perf issues (at the time of writing 😉).
UPDATE2: Seems like it is the callFetches function that causes the perf degredation