When I ran your snippet with 80 rows of data, there wasn't any time difference in execution of the whole macro until Sleep with a value of 8 was used. Until then, there was no significant difference in execution time from when that line was commented out altogether. Granted mine's not the newest machine on the block.
I also needed either to put an Application.screenupdating =True or a DoEvents within the loop, else the charts didn't update visually.
So with those I couldn't get faster than 1 chart per 40 milliseconds.
  Set myrng = Sheets("x").Cells(1).CurrentRegion
  For rw = 1 To myrng.Rows.Count Step 20
    With .SeriesCollection(1)
      .XValues = myrng.Rows(rw)
      .Values = Sheets("Y").Cells(rw, 1).Resize(, myrng.Columns.Count)
    End With
    Sleep (200)
    Application.ScreenUpdating = True
    'DoEvents
  Next rw