public with sharing class MyDataController {
    @AuraEnabled
    public static String[] getStringData() {
      String[] myDataList = new List<String>();
      for(Integer i = 0; i < 2000; i++) {
        myDataList.add('element_' + i);
      }
      return myDataList;
    }
  }

<aura:application controller="MyDataController">
  <aura:attribute name="myDataList" type="String[]" />

  <aura:handler name="init" value="{!this}" action="{!c.onInit}" />

  <button onclick="{!c.onButtonClick}">Click</button>
</aura:application>

({
    onInit: function(component, event, helper) {
        var action = component.get("c.getStringData");
        action.setCallback(this, function(response) {
            if (response.getState() === "SUCCESS") {
                component.set("v.myDataList", response.getReturnValue());
            }
        });

        $A.enqueueAction(action);
    },

    onButtonClick: function(component, event, helper) {
        var dataList = component.get("v.myDataList");
        var element;
        var i = 0;
        console.time("loop_time");
        for(i; i < 2000; i++) {
            element = dataList[i];
        }
        console.timeEnd("loop_time");
    }
})

onButtonClick: function(component, event, helper) {
    var dataList = component.get("v.myDataList");
    var element;
    console.time("loop_time");
    dataList.forEach(function(item){
        element = item;
    });
    console.timeEnd("loop_time");
}

@AuraEnabled
public static String getStringData() {
  String[] myDataList = new List<String>();
  for(Integer i = 0; i < 2000; i++) {
    myDataList.add('element_' + i);
  }
  return JSON.serialize(myDataList);
}

<aura:attribute name="myDataString" type="String" default="" />

onInit: function(component, event, helper) {
    var action = component.get("c.getStringData");
    action.setCallback(this, function(response) {
        if (response.getState() === "SUCCESS") {
             component.set("v.myDataString", response.getReturnValue());
        }
    });
    $A.enqueueAction(action);
},

onButtonClick: function(component,event,helper) {
    var temp = component.get("v.myDataString");
    var dataList = JSON.parse(temp);
    var element;
    console.time("loop_time");
    dataList.forEach(function(item){
        element = item;
    });
    console.timeEnd("loop_time");
}