Filtering a large collection (28000 members) as the user types

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C#

WPF

collections

DataGrid

What I have attempted to do is filter an ObservableCollection of Companies as the user is typing into a TextBox. The user is able to check CheckBoxes to add further criteria to what they would like to search on.

Sorry if this is a particularly long example but as I said, the method is very slow at the moment and I am looking for ways to improve it.

What I have tried:

The main issue with what I have tried so far is that it is really,really slow when the user types into the SearchBox. Here is how I apply the filter first of all;

CollectionViewSource.GetDefaultView(Companies).Filter = FilterCompanies;

When the user types into the TextBox this method is called;

private void SearchBoxCallApplyFilters(object sender, TextChangedEventArgs e)
{
    CollectionViewSource.GetDefaultView(Companies).Refresh();
}

And then the FilterCompanies method;

private bool FilterCompanies(object obj)
{
    var company = obj as CompanyModel;
    if (searchBox.Text == string.Empty)
    {
        return true;
    }
    else if (CharactersOnly(company.Name.ToLower()).Contains((CharactersOnly(searchBox.Text).ToLower()))
        || CharactersOnly(company.Town.ToLower()).Contains((CharactersOnly(searchBox.Text).ToLower()))
        || CharactersOnly(company.Postcode.ToLower()).Contains((CharactersOnly(searchBox.Text).ToLower())))
    {
        if (FilterByCheckBoxes(company))
        {
            return true;
        }

    }
    return false;
}

And finally the FilterByCheckBoxes method;

private bool FilterByCheckBoxes(CompanyModel company)
{
    if (currentCheckBox.IsChecked == true && company.CurrentStatus == 1)
    {
        return true;
    }
    if (subbieCheckBox.IsChecked == true && company.Subcontractor == 1)
    {
        return true;
    }
    if (supplierCheckBox.IsChecked == true && company.Supplier == 1)
    {
        return true;
    }
    if (planthireCheckBox.IsChecked == true && company.Planthire == 1)
    {
        return true;
    }
    if (architectCheckBox.IsChecked == true && company.Architect == 1)
    {
        return true;
    }
    if (qsCheckBox.IsChecked == true && company.QS == 1)
    {
        return true;
    }
    if (projectManagerCheckBox.IsChecked == true && company.ProjectManager == 1)
    {
        return true;
    }
    if (structEngCheckBox.IsChecked == true && company.StructEng == 1)
    {
        return true;
    }
    if (servEngCheckBox.IsChecked == true && company.ServiceEng == 1)
    {
        return true;
    }
    return false;
}

CharactersOnly simply removes anything that isn't a Character from the TextBox Text;

private string CharactersOnly(string input)
{
    char[] arr = input.ToCharArray();
    arr = Array.FindAll(arr, (c => (char.IsLetterOrDigit(c))));
    return new string(arr);
}