Do you only get the rows you need?

It's expensive retrieving data from a database, as such it's important to only ask for the rows you require when getting data.

Entity Framework nicely translates the various filters like the Where method into SQL WHERE clauses. This makes it really easy to write nice readable code that is also very efficient.

List<Sale> sales = context.Sales.ToList();
foreach (var sale in sales)
{
    if (sale.ProductId == request.ProductId)
    {
        // Do stuff
    }
}

List<Sale> sales = context.Sales
      .Where(sale => sale.ProductId == Request.ProductId)
      .ToList();
      
foreach (var sale in sales)
{
    // Do stuff
}