Using a for loop and adding items that match the criteria to a new list can often be more performant than LINQ for large lists:

var filteredList = new List<MyObject>();
for (int i = 0; i < myObjects.Count; i++)
{
    var o = myObjects[i];
    if (o.Value > 100 && o.Date < DateTime.Now.AddDays(-7) && SomeComplexCalculation(o) > 50)
    {
        filteredList.Add(o);
    }
}

Consider optimizing SomeComplexCalculation itself for better overall performance. If appropriate, you could potentially pre-calculate results of SomeComplexCalculation and store them as a property on MyObject to avoid repeated expensive computations during filtering.