Removing checks that are only necessary on first pass
You can remove the check for RawBody.IsNull() because you don't read the value of RawBody inside this method. If at the other places where some similiar code is called the rawBody is read, I would suggest to extract this check to a separate method and setting a flag which indicates that the check is already done. Like  

private bool _bodyValueIsAssigned = false;
private void AssignValueIfNotAssigned(Frame frame)
{
    if (_bodyValueIsAssigned) { return; }

    if (RawBody.IsNull())
    {
        RawBody = frame;
    }

    if (FilteredBody.IsNull())
    {
        FilteredBody = frame;
    }

    _bodyValueIsAssigned  = true;
}


This can be called for the point where you call the Update() method. Without seeing more about how this is called (from an event or something similiar) there can't be said more.  



If Filters is a List&lt;T&gt; or an array you should use performance wise a for loop instead of a foreach. See: https://stackoverflow.com/a/365658