Speed up javascript code to convert csv to geojson
From a short review;

Old school for loops still beat map

Prevent repeated object[string] access, like accesssing several times item[latName], because it is relatively slow

Is it absolutely necessary to create a new object? I have a suspicion that modifying the object would be faster. (Yes, not great practice, but you are looking for speed, you should try this)

One more thing, I was hesitant to mention this but, but the plus operator can be faster than parseFloat according to jsperf. So you could just go for
  const newLat = +item[latName];
  const newLong = +item[longName];



give it a shot, let us know ;)
This is my counter proposal with in place conversion;
function convertLocationToNumbers(data, latName, longName) {
  const dataLength = data.length;
  for(let i = 0; i &lt; dataLength; i++){
    const item = data[i];
    //Dont check for hasownproperty, just go!
    const newLat = +item[latName];
    const newLong = +item[longName];
    if(isNaN(newLat) || isNaN(newLong ){
      item[latName] = 42.2; 
      item[lonName] = ~83.6;
    }else{
      item[latName] = newLat; 
      item[lonName] = newLong;
    } 
  }
}