How to make an iteration in a for-loop faster?

I have a list with contentlets and want all languages from every contentlet (I get these with the method languageAPI.getAllValuesByKey(key , contentList). I get a HashMap and iterate over it. There are 1000 keys available. In the beginning it only takes 2 ms per key. But after a while it increases. And the last one takes 35ms. How can I decrease these times? How can I make it faster/more efficient?

JSONArray arrAll = new JSONArray();
JSONObject jsonObject = new JSONObject();
JSONArray arr = new JSONArray();
JSONObject values = new JSONObject();

List<Contentlet> contentlets = languageFactory.getAllContentlets(null);
List<Contentlet> keys = languageAPI.getLanguageKeys(null, contentlets);

for(Contentlet key : keys) {
    Long startTime = System.currentTimeMillis();
    jsonObject = new JSONObject();

    HashMap<Long, String> allValues = languageAPI.getAllValuesByKey(key.getStringProperty("key"), contentlets);
    Iterator<java.util.Map.Entry<Long, String>> it = allValues.entrySet().iterator();
    arr = new JSONArray();
    while (it.hasNext()) {
        values = new JSONObject();
        java.util.Map.Entry<Long, String> pairs = it.next();
        values.put("l", pairs.getKey());
        values.put("v", pairs.getValue());
        arr.add(values);
        it.remove(); // avoids a ConcurrentModificationException
    }

    try {
        jsonObject.put("k", key.getStringProperty("key"));
        jsonObject.put("t", (Object)arr);
        jsonObject.put("p", key.isLive());
        jsonObject.put("l", key.isLocked());
        jsonObject.put("a", key.isArchived());
    } catch (DotStateException e) {
        throw new RuntimeException(e.toString(),e);
    } catch (DotDataException e) {
        throw new RuntimeException(e.toString(),e);
    } catch (DotSecurityException e) {
        throw new RuntimeException(e.toString(),e);
    }

    arrAll.add(jsonObject);
    Long end = System.currentTimeMillis() - startTime;
    Logger.info(this, "For key: " + key.getStringProperty("key") + " " + end  + "ms");
}

Edit:

One of the problems is that the for loop in the getStringKey method take some time. Of course in the beginning the value is at the beginning, but after a while, it is at the end of the list. So I think one of the problems might be here (this takes 8ms, in the last records)

public HashMap<Long, String> getAllValuesByKey(String key, List<Contentlet> contentlets) {
    HashMap<Long, String> keys = new HashMap<Long, String>();
    for(Language language : APILocator.getLanguageAPI().getLanguages()) {
        keys.put(language.getId(), getStringKey(language.getId(), key, contentlets));
    }
    return keys;
}

public String getStringKey(Long languageId, String key, List<Contentlet> contentlets){
    String value=null;

    for(Contentlet keyEntry : contentlets) {
        if(keyEntry.getStringProperty("key").equals(key) && languageId == keyEntry.getLanguageId()) {
            return keyEntry.getStringProperty("value");
        }
    }

    if(value==null)
    {
        value = "";
    }
    return value;
}   

Edit 2: found the problem, but how to solve it?

The code after this phrase takes 20ms in the last records. The only thing what happens here is adding some things to a JSONObject. Why is this taking so long? I even don't know, how to make this faster, because this is the only way to handle it?

    end = System.currentTimeMillis() - startTime;
    Logger.info(this, "Before add: " + key.getStringProperty("key") + " " + end  + "ms");
    try {
        jsonObject.put("k", key.getStringProperty("key"));
        jsonObject.put("t", (Object)arr);
        jsonObject.put("p", key.isLive());
        jsonObject.put("l", key.isLocked());
        jsonObject.put("a", key.isArchived());
    } catch (DotStateException e) {
        throw new RuntimeException(e.toString(),e);
    } catch (DotDataException e) {
        throw new RuntimeException(e.toString(),e);
    } catch (DotSecurityException e) {
        throw new RuntimeException(e.toString(),e);
    }
    end = System.currentTimeMillis() - startTime;
    Logger.info(this, "After add: " + key.getStringProperty("key") + " " + end  + "ms");