PagedList with large data sets

Question

I'm using X.PagedList for paging functionality in my application. The code I have works fine but when I return larger sets of data it seems like it could be faster. Is there anything I can do to my code to have it perform more efficiently?

Controller Code

var employees = _service.GetEmployeesByOffice(region, area, office).OrderBy(e => e.FullName);
var model = Mapper.Map<IEnumerable<Employee>,IEnumerable<EmployeesViewModel>>(employees).ToPagedList(page, 20);

var viewModel = new EmployeeListingView
{
    EmployeeViewModel = model,
    // Additional...
};

return View(viewModel);

Service Call

public IEnumerable<Employee> GetEmployeesByOffice(string region, string area, string office)
{
    var query = _context.Employees.AsQueryable();

    if (!String.IsNullOrEmpty(region))
        query = query.Where(e => e.Region == region);

    if (!String.IsNullOrEmpty(area))
        query = query.Where(e => e.Area == area);

    if (!String.IsNullOrEmpty(office))
        query = query.Where(e => e.Office == office);

    return query;
}

Razor Page

@Html.PagedListPager((IPagedList)Model.EmployeeViewModel, page => Url.Action("Index", new { area = Model.SelectedArea, office = Model.SelectedOffice, page }))

Answer 1

I cannot say anything about the mapping but you can simplify this query

public IEnumerable<Employee> GetEmployeesByOffice(string region, string area, string office)
{
    var query = _context.Employees.AsQueryable();

    if (!String.IsNullOrEmpty(region))
        query = query.Where(e => e.Region == region);

    if (!String.IsNullOrEmpty(area))
        query = query.Where(e => e.Area == area);

    if (!String.IsNullOrEmpty(office))
        query = query.Where(e => e.Office == office);

    return query;
}

into this one. Each expression will resove either to true or the actual condition if the variable was provided.

public IEnumerable<Employee> GetEmployeesByOffice(string region, string area, string office)
{
    return
        from e in _context.Employees.AsQueryable()
        where
            (string.IsNullOrEmpty(region) || e.Region == region) &&
            (string.IsNullOrEmpty(area) || e.Area == area) &&
            (string.IsNullOrEmpty(office) || e.Office == office);
}

Answer 2

So it seems that the issue was with the mapping. Using this answer, I created an extension to handle mapping the object after calling it. I changed this code

var employees = 
      _service
          .GetEmployeesByOffice(region, area, office)
          .OrderBy(e => e.FullName);
var model = 
      Mapper.Map<IEnumerable<Employee>, IEnumerable<EmployeesViewModel>>(employees)
      .ToPagedList(page, 20);

to this

var model = 
    _service
        .GetEmployeesByOffice(region, sArea, sOffice)
        .OrderBy(e => e.FullName)
        .ToPagedList(page, 20)
        .ToMappedPagedList<Employee, EmployeesViewModel>();