MVC 5 MultiThreading - asp.net-mvc

Question

I'm trying to setup multithreading in a .NET MVC application. I am processing a large amount of data, so my hope is if I split it into multiple threads I can speed up the processing time. The method isn't waiting for the tasks to complete though before it runs the data, so it's currently returning an empty list. I want to wait for all of the tasks to be completed before it returns the final result.
If there's a better way to this I'm loading a datatable and basically want to take the roster variable and run the CheckCompliance method for each individual. Without multithreading the page could take minutes to load, I'm wanting the page to load with almost no delay.
public async Task<Tuple<List<RosterListing>, int, int>> Search(int employerId, DataTableAjaxPostModel model)
{
List<RosterListing> roster = _service.Search(employerId, searchBy, columnSearches, take, skip, sortBy, sortDir, out int filteredResultsCount, out int totalResultsCount);
IEnumerable<Mandate> enforcedMandates = _mandateService.GetEnforcedMandates();
IEnumerable<IndividualMandateData> individualMandateData = _individualMandateDataService.GetAllForPtbIds(roster.Select(x => Convert.ToInt32(x.PtbId)));
List<RosterListing> newRoster = new List<RosterListing>();
// Split main list into sections for multithreading
List<List<RosterListing>> chunks = roster.ChunkBy(5);
var tasks = new List<Task<List<RosterListing>>>();
foreach (var chunk in chunks)
{
tasks.Add(Task.Run(async () =>
{
List<RosterListing> updatedRoster = new List<RosterListing>();
updatedRoster.AddRange(await CheckCompliance(chunk, enforcedMandates, individualMandateData));
return updatedRoster;
}));
}
var results = Task.WhenAll(tasks);
if (results.Status == TaskStatus.RanToCompletion)
{
foreach (var result in results.Result)
{
newRoster.AddRange(result);
}
}
// This doesn't wait until all tasks are complete
return await Task.Run(() => Tuple.Create(newRoster, filteredResultsCount, totalResultsCount));
}
private async Task<List<RosterListing>> CheckCompliance(List<RosterListing> roster, IEnumerable<Mandate> enforcedMandates, IEnumerable<IndividualMandateData> individualMandateData)
{
foreach (var officer in roster)
{
foreach (var mandate in enforcedMandates)
{
IndividualMandateData data = individualMandateData.FirstOrDefault(x => x.MandateId == mandate.MandateId && x.PtbId == Convert.ToInt32(officer.PtbId));
if (data == null)
{
officer.IsInCompliance = false;
break;
}
if (mandate.MandateRequirementUnit == "Course")
{
if (data.HoursAttended < 0)
{
officer.IsInCompliance = false;
break;
}
}
else
{
if (data.HoursAttended < mandate.Requirement)
{
officer.IsInCompliance = false;
break;
}
}
officer.IsInCompliance = true;
}
}
return await Task.Run(() => roster);
}