DataGridView тормозит

Imports System.IO
Imports System.Text
Public Class Form3
    Private cache As Cached_File
    Private Sub Form3_Load(sender As System.Object, e As System.EventArgs) Handles MyBase.Load
        'если нет файла для тестирования используйте для его формирования CreateFileIfNeed
        Dim file = "c:\temp345.txt"  '400 mb
        cache = New Cached_File()
        cache.OpenFile(file, vbTab)
        With DataGridView1
            .Parent = Me
            .Dock = DockStyle.Fill
            .VirtualMode = True
            .AllowUserToAddRows = False
            .ReadOnly = True
            .RowCount = cache.LinesCount
            .ColumnCount = cache(0).Length
        End With
    End Sub
    Private Sub DataGridView1_CellValueNeeded(sender As System.Object, e As System.Windows.Forms.DataGridViewCellValueEventArgs) Handles DataGridView1.CellValueNeeded
        InlineAssignHelper(e.Value, cache(e.RowIndex)(e.ColumnIndex))
    End Sub
    Private Function InlineAssignHelper(Of T)(ByRef target As T, value As T) As T
        target = value
        Return value
    End Function
    'Private Sub CreateFileIfNeed(filePath As String)
    '    If Not File.Exists(filePath) Then
    '        Dim rnd = New Random()
    '        Dim sb = New StringBuilder()
    '        Using bw = New StreamWriter(filePath)
    '            For i As Integer = 0 To 199999
    '                sb.Length = 0
    '                sb.AppendFormat(i.ToString())
    '                For j As Integer = 0 To 255
    '                    sb.Append(vbTab + rnd.Next(10000000))
    '                Next
    '                bw.WriteLine(sb.ToString())
    '            Next
    '        End Using
    '    End If
    'End Sub
End Class
 
Public Class Cached_File
    Private linePositions As New List(Of UInteger)
    Private loadedLines As New List(Of String())
    Private firstLoadedLineIndex As Integer
    Const preLoadCount As Integer = 100
    Private filePath As String
    Private separator As Char
    Public ReadOnly Property LinesCount() As Integer
        Get
            Return linePositions.Count
        End Get
    End Property
    Public Sub OpenFile(fPath As String, sep As Char)
        filePath = fPath
        separator = sep
        linePositions.Clear()
        linePositions.Add(0)
        Using fs = New FileStream(filePath, FileMode.Open)
            Dim length = fs.Length
            For i As Integer = 0 To length - 1
                If fs.ReadByte() = Asc(ControlChars.Lf) Then
                    linePositions.Add(CUInt(fs.Position + 1))
                End If
            Next
        End Using
        linePositions.RemoveAt(linePositions.Count - 1)
        Load(0)
    End Sub
    Private Sub Load(iLine As Integer)
        Dim from = Math.Max(0, iLine - preLoadCount)
        Dim [to] = Math.Min(from + preLoadCount * 2, linePositions.Count - 1)
        Dim count = [to] - from + 1
        loadedLines.Clear()
        Using sr = New StreamReader(filePath, Encoding.Default)
            sr.BaseStream.Seek(linePositions(from), SeekOrigin.Begin)
            For i As Integer = 0 To count - 1
                loadedLines.Add(sr.ReadLine().Split(separator))
            Next
        End Using
        firstLoadedLineIndex = from
    End Sub
    Default Public ReadOnly Property Item(iLine As Integer) As String()
        Get
            If iLine < 0 OrElse iLine >= linePositions.Count Then
                Throw New IndexOutOfRangeException()
            End If
            If iLine < firstLoadedLineIndex OrElse iLine >= firstLoadedLineIndex + loadedLines.Count Then
                Load(iLine)
            End If
            Return loadedLines(iLine - firstLoadedLineIndex)
        End Get
    End Property
End Class