Is your feature request related to a problem or challenge? Please describe what you are trying to do.
While reviewing #7263 from @zhuqi-lucas I noticed that the json reader is allocating a new for each string field it parses

Describe the solution you'd like

I would like to make the json reader faster by not allocating in the inner loop

Describe alternatives you've considered

I think instead of doing

for ... {
  builder.append_value(n.to_string())
}

A typically faster pattern is

// temp buffer
let mut s = String::new();

for ... {
  s.clear(); // reuse allocation
  write!(&mut s, "{n}");
  builder.append_value(s)
}

Additional context