Fast string formatting in for loop

I have a function that loops through large objects of raw data, translating coded strings into readable words. The function is very basic, and I keep wondering if I should use an object instead containing the words to be detected and translated. This would be cleaner-looking and more flexible, but it would not like it to harm the performance of the script, since it needs to run through potentially thousands of items.

function formatTitle(input) {
    var input_lowercase = input.toLowerCase() 
    if (input == "total") {
        return "Title"
    } else if ( input_lowercase == "type" || input_lowercase == "dokumenttype" ||  input_lowercase == "km_type"  ) {
        return label_documenttype
    } else if ( input_lowercase.indexOf("tema_") >= 0 || input_lowercase == "spesfikasjon" ) {
        return label_theme
    } else if ( input_lowercase.indexOf("prosess_") >= 0) {
        return label_process   
    } else if (input.indexOf("SM_") >= 0) {
        return input.substr(3);           
    } 
    else {
        return input.charAt(0).toUpperCase() + input.slice(1)
    }
}

There are more exceptions in the list, I've shortened it a bit down for practical reasons. I use the function in the following context (when t_formatheader is switched on):

if ( skipping_mechanism == "lookup" ) { //42% faster than switch block in IE
        for (var key in arr_firstnode) {                
            if (key in keysToIgnore) continue;  
            if ( t_formatheader == "1" ) {
                arrayPushUnknown( columns, formatTitle( key ), "", true, key )
            }
            else {
                arrayPushUnknown( columns, key, "", true, key )     
            }
        }
    }

I think you need a mix of lookup object(s) and regular if statements; there are clearly direct matches, matches on the prefix and the rest. Also, to write else if is pointless since you return anyway in previous if checks, so you can simply use

if(condition){
  return value
}
if(condition2){ // <- no else here
  return value2
}

All that together you could consider something like this:

var formatTitleDirectMapping = {
  'total' : 'title',
  'type' : label_documenttype,
  'dokumenttype' : label_documenttype,
  'km_type' : label_documenttype,
  'spesfikasjon' : label_theme,
}

var formatTitlePrefixMapping = {
  'tema_' : label_theme,
  'prosess_' : label_process,
  'dokumenttype' : label_documenttype,
  'km_type' : label_documenttype,
}    

function formatTitle(input) {
  var lower = input.toLowerCase();
  //Is there a straight forward conversion? Then get out
  if( formatTitleDirectMapping[lower] )
    return formatTitleDirectMapping[lower];
  //Is there a straight forward conversion for the prefix? Then get out
  var prefix = lower.split("_")[0];
  if( formatTitlePrefixMapping[prefix] )
    return formatTitlePrefixMapping[prefix];
  //Put the conversions that dont lend themselves to lookup structures here
  if( prefix == "sm") {
    return input.substr(3);           
  } 
  //Catch all
  return input.charAt(0).toUpperCase() + input.slice(1)
}

StackExchange Code Review Q#60714, answer score: 3