CouchDB - Filtered Replication - Can the speed be improved?

I have a single database (300MB & 42,924 documents) consisting of about 20 different kinds of documents from about 200 users. The documents range in size from a few bytes to many KiloBytes (150KB or so).

When the server is unloaded, the following replication filter function takes about 2.5 minutes to complete. When the server is loaded, it takes >10 minutes.

Can anyone comment on whether these times are expected, and if not, suggest how I might optimize things in order to get better performance?

function(doc, req) {
    acceptedDate = true;
    if(doc.date) {
        var docDate = new Date();
        var dateKey = doc.date;
        docDate.setFullYear(dateKey[0], dateKey[1], dateKey[2]);

        var reqYear = req.query.year;
        var reqMonth = req.query.month;
        var reqDay = req.query.day;
        var reqDate = new Date();
        reqDate.setFullYear(reqYear, reqMonth, reqDay);

        acceptedDate = docDate.getTime() >= reqDate.getTime();
    }

    return doc.user_id && doc.user_id == req.query.userid && doc._id.indexOf("_design") != 0 && acceptedDate; 
}