My computer Environment:
Core i7 2630QM
DDR3 8GB memory
Windows 8.1 x64

I create a 1048576*20 cells worksheet, and compare it others, memory usage and loading
speed is the following:

                                   [Create Directly]        [Load file]
          Devexpress         Out of memory          1233 MB  83s       
           Infragistics         583 MB                     618 MB   130s
MS Excel 2013 x64                                         390 MB   30s

Load speed of different Excel document may be vary, it may be very difficult to optimize all cases.

Thank you.

================================================================

<Window
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:ig="http://schemas.infragistics.com/xaml" x:Class="InfSpread.MainWindow"
        Title="Infragistics Spreadsheet Test" Height="480" Width="640">
    <Grid>

        <Grid.RowDefinitions>
            <RowDefinition Height="Auto"/>
            <RowDefinition Height="*"/>
        </Grid.RowDefinitions>

        <StackPanel Orientation="Horizontal">
            <Button Margin="5" Content="Create Directly" Click="Create_OnClick"/>
            <Button Margin="5" Content="Create File" Click="CreateFile_OnClick"/>
            <Button Margin="5" Content="Load File" Click="Load_OnClick"/>
        </StackPanel>

        <ig:XamSpreadsheet x:Name="spread" Grid.Row="1"/>

    </Grid>
</Window>

using System;
using System.Collections.Generic;
using System.Diagnostics;
using System.IO;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows;
using System.Windows.Controls;
using System.Windows.Data;
using System.Windows.Documents;
using System.Windows.Input;
using System.Windows.Media;
using System.Windows.Media.Imaging;
using System.Windows.Navigation;
using System.Windows.Shapes;
using Infragistics.Documents.Excel;
using Microsoft.Win32;

namespace InfSpread
{
    /// <summary>
    /// Interaction logic for MainWindow.xaml
    /// </summary>
    public partial class MainWindow : Window
    {
        public MainWindow()
        {
            InitializeComponent();
        }

        private void Create_OnClick(object sender, RoutedEventArgs e)
        {
            var workbook = spread.Workbook;
            workbook.SetCurrentFormat(WorkbookFormat.Excel2007);
            spread.IsUndoEnabled = false;
            FillWorksheet(spread.ActiveWorksheet, 1048576);
            spread.IsUndoEnabled = true;
        }

        private void CreateFile_OnClick(object sender, RoutedEventArgs e)
        {
            var dlg = new SaveFileDialog();
            dlg.DefaultExt = ".xlsx";
            dlg.Filter = "Excel documents (.xlsx)|*.xlsx";

            var result = dlg.ShowDialog();

            if (result == true)
            {
                var file = dlg.FileName;

                // Create a Workbook object
                var workbook = new Workbook();
                workbook.SetCurrentFormat(WorkbookFormat.Excel2007);
                var sheet = workbook.Worksheets.Add("Sheet0");
                FillWorksheet(sheet, 1048576);

                using (Stream stream = File.Open(file, FileMode.OpenOrCreate))
                {
                    workbook.Save(stream);
                }
            }
        }

        private void Load_OnClick(object sender, RoutedEventArgs e)
        {
            var dlg = new OpenFileDialog();
            dlg.DefaultExt = ".xlsx";
            dlg.Filter = "Excel documents (.xlsx)|*.xlsx";

            var result = dlg.ShowDialog();

            if (result == true)
            {
                var file = dlg.FileName;
                var workbook = Workbook.Load(file);
                if (workbook != null)
                {
                    spread.Workbook = workbook;
                }
            }
        }

        private void FillWorksheet(Worksheet sheet, int numberOfRows)
        {
            var sw = new Stopwatch();
            sw.Start();

            for (int index = 0; index < numberOfRows; index++)
            {
                var row = sheet.Rows[index];
                var timeSpan = sw.Elapsed;
                row.SetCellValue(0, string.Format("{0:X8}", index));
                row.SetCellValue(1, timeSpan.Hours * 10000000 + timeSpan.Minutes * 100000 + timeSpan.Seconds * 1000 + timeSpan.Milliseconds);
                row.SetCellValue(2, timeSpan.TotalMilliseconds);
                row.SetCellValue(3, timeSpan.Ticks);
                row.SetCellValue(4, "F");
                row.SetCellValue(5, "400");
                row.SetCellValue(6, "50");
                row.SetCellValue(7, "50, 30, 567");
                row.SetCellValue(8, "2015-12-12");
                row.SetCellValue(9, "Feature Co. Ltd.");
                row.SetCellValue(10, "Star War Plan");
                row.SetCellValue(11, "+1-212-123-1234");
                row.SetCellValue(12, "0123456789-1234567890");
                row.SetCellValue(13, "abcdefgabcdefg hijklnm");
                row.SetCellValue(14, "OPQRST UVWXYZ");
                row.SetCellValue(15, "Description: For free use");
                row.SetCellValue(16, "Indoors use only");
                row.SetCellValue(17, "Maybe you know world trip is a nice thing, o ha ...");
                row.SetCellValue(18, "E-Mail: your.name@outlook.com");
                row.SetCellValue(19, "Say something, I feel lonely, thank you.");

                if (index % 1000 == 0)
                {
                    Console.WriteLine(index);
                }

            }

            sw.Stop();
        }
    }
}

============================================================

<Window
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:dxsps="http://schemas.devexpress.com/winfx/2008/xaml/spreadsheet" x:Class="DevSpread.MainWindow"
        Title="Devexpress Spreadsheet Test" Height="480" Width="640">
    <Grid>

        <Grid.RowDefinitions>
            <RowDefinition Height="Auto"/>
            <RowDefinition Height="*"/>
        </Grid.RowDefinitions>

        <StackPanel Orientation="Horizontal">
            <Button Margin="5" Content="Create Directly" Click="Create_OnClick"/>
            <Button Margin="5" Content="Create File" Click="CreateFile_OnClick"/>
            <Button Margin="5" Content="Load File" Click="Load_OnClick"/>
        </StackPanel>

        <dxsps:SpreadsheetControl x:Name="spread" Grid.Row="1"/>
    </Grid>
</Window>

using System;
using System.Collections.Generic;
using System.Diagnostics;
using System.IO;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows;
using System.Windows.Controls;
using System.Windows.Data;
using System.Windows.Documents;
using System.Windows.Input;
using System.Windows.Media;
using System.Windows.Media.Imaging;
using System.Windows.Navigation;
using System.Windows.Shapes;
using DevExpress.Spreadsheet;
using Microsoft.Win32;

namespace DevSpread
{
    /// <summary>
    /// Interaction logic for MainWindow.xaml
    /// </summary>
    public partial class MainWindow : Window
    {
        public MainWindow()
        {
            InitializeComponent();
        }

        private void Create_OnClick(object sender, RoutedEventArgs e)
        {
            spread.BeginUpdate();
            FillWorksheet(spread.ActiveWorksheet, 1048576);
            spread.EndUpdate();
        }

        private void CreateFile_OnClick(object sender, RoutedEventArgs e)
        {
            var dlg = new SaveFileDialog();
            dlg.DefaultExt = ".xlsx";
            dlg.Filter = "Excel documents (.xlsx)|*.xlsx";

            var result = dlg.ShowDialog();

            if (result == true)
            {
                var file = dlg.FileName;

                spread.BeginUpdate();
                FillWorksheet(spread.ActiveWorksheet, 1048576);
                spread.EndUpdate();

                spread.SaveDocument(file);
            }
        }

        private void Load_OnClick(object sender, RoutedEventArgs e)
        {
            var dlg = new OpenFileDialog();
            dlg.DefaultExt = ".xlsx";
            dlg.Filter = "Excel documents (.xlsx)|*.xlsx";

            var result = dlg.ShowDialog();

            if (result == true)
            {
                var file = dlg.FileName;
                spread.Document.LoadDocument(file);
            }
        }

        private void FillWorksheet(Worksheet sheet, int numberOfRows)
        {

            var sw = new Stopwatch();
            sw.Start();
            spread.BeginUpdate();

            for (int index = 0; index < numberOfRows; index++)
            {
                var row = sheet.Rows[index];

                row[0].Value = string.Format("{0:X8}", index);
                var timeSpan = sw.Elapsed;
                row[1].Value = timeSpan.Hours * 10000000 + timeSpan.Minutes * 100000 + timeSpan.Seconds * 1000 + timeSpan.Milliseconds;
                row[2].Value = timeSpan.TotalMilliseconds;
                row[3].Value = timeSpan.Ticks;

                row[4].Value = "F";
                row[5].Value = "400";
                row[6].Value = "50";
                row[7].Value = "50, 30, 567";
                row[8].Value = "2015-12-12";
                row[9].Value = "Feature Co. Ltd.";
                row[10].Value = "Star War Plan";
                row[11].Value = "+1-212-123-1234";
                row[12].Value = "0123456789-1234567890";
                row[13].Value = "abcdefgabcdefg hijklnm";
                row[14].Value = "OPQRST UVWXYZ";
                row[15].Value = "Description: For free use";
                row[16].Value = "Indoors use only";
                row[17].Value = "Maybe you know world trip is a nice thing, o ha ...";
                row[18].Value = "E-Mail: your.name@outlook.com";
                row[19].Value = "Say something, I feel lonely, thank you.";

                if (index % 1000 == 0)
                {
                    Console.WriteLine(index);
                }

            }
            spread.EndUpdate();
            sw.Stop();
        }
    }
}