Grouping two sets of data The following method retrieves a list of users and the subscriptions for each user, from my database. That said, it works, but it runs pretty dang slow. I'd like to improve the speed on it. Currently, it fetches a mere 90 records, and takes nearly 20 seconds to do so, partly because, for every record of final output, it's grouping the subs per user, and doing a fetch on that user to obtain userId, email, and username (all three as User below).

public IEnumerable&lt;ActiveSubscriptionsSummary&gt; ActiveSubscriptionsByUser(SubscriptionType type)
{
    var subs = new List&lt;ActiveSubscriptionsSummary&gt;();

    var subscriptions = _bss.ActiveSubscriptions(type);
    var subsByUser = subscriptions
        .GroupBy(s =&gt; s.UserId,
            s =&gt; s,
            (key, g) =&gt; new ActiveBuilderSubscriptionsSummary
            {
                User = _cs.ById(key),
                Subscriptions = g.ToList()
            })
        .ToList();

    return subsByUser;
}


_cs.ById above refers to a service call that retrieves the user:

public ICustomer ById(Guid id)
{
    return GetBackend().ById(id);
}


All I use from that method is UserId, UserName, and Email (in the ActiveSubscriptionsByUser method, that is).

_bss above refers to a service call, with this method:

public IEnumerable&lt;Subscription&gt; ActiveSubscriptions(string subscriptionType)
{
    var subs = _sms.CurrentSession.Query&lt;Subscription&gt;()
                    .Where(s =&gt; (s.SubscriptionType == subscriptionType) &amp;&amp;
                                (s.EndDate &gt;= DateTime.Today.Date))
                    .ToList();
    return subs;
}


My goal here is to speed this thing up, as I know over time that "90" will be a pretty small number, and we'll potentially be showing 100's, if not 1000's, of subs on the screen. And 20 seconds for 90 equates to a long dang time for 1000+.

Is my GroupBy potentially something that can be better coded to make this run faster? Could I potentially join two queries "users" and "subs" in some better way?

Update
This is C#, running against an nHibernate-hooked SQL DB, outputting to Angular.
Your performance is going to suffer until you can create a "WHERE IN"-style query for users, either by building a string or by combining Expressions. 

Or, if you want to keep it simple, just put the active criteria in your query, don't pull it up front. I'd recommend this approach.

Add .Expand("Users") on the active subscriptions query to pull back the related Users. I would avoid getting your ActiveBuilderSubscriptionSummary involved until you get your results back, at which point you can then project the query results to a collection of ActiveBuilderSubscriptionSummary objects using a standard LINQ .Select.

If you are hesitant to remove or change your method because you need to reuse it, or it is comfortably nestled in a repository, I'd refactor to use a stored proc instead. Otherwise, faced with the choice, it is way better to duplicate your query in code than to force an eager load of data up front.