Most development time is going to be spent on reading code that's already written, so yes, they do matter. With the speeds mentioned it's not going to be appreciable until you hit a massive scale, which, let's face it, most of us aren't working with.
const injectedValue = JSON.parse("$SERVER_JSON_VALUE.replace("\"","\\\"")");
// vs
const injectedValue = $SERVER_JSON_VALUE;
generally for a single value in the codebase is emphatically NOT a huge issue... and if it saves 80-120ms or so on the load, that's a significant impact. Not to mention the lower memory overhead while doing so.
