This also speeds up situations where you only need the fragments/toc and not the rendered content, e.g. Related
with fragments type indexing:
```bash
name old time/op new time/op delta
RelatedSite-10 12.3ms ± 2% 10.7ms ± 1% -12.95% (p=0.029 n=4+4)
name old alloc/op new alloc/op delta
RelatedSite-10 38.6MB ± 0% 38.2MB ± 0% -1.08% (p=0.029 n=4+4)
name old allocs/op new allocs/op delta
RelatedSite-10 117k ± 0% 115k ± 0% -1.36% (p=0.029 n=4+4)
```
Fixes#10750
271318ad78