How can I improve performance for my JavaScript table class?

I've written a JavaScript table class to make it easy to display tabular data and allow the user to sort the data by clicking on the table headers.

However, when there are hundreds of rows, performance on the sorting is a bit sluggish. I would like to know what kind of improvements I can make. I suspect that my string concatenation methods in displayData may not be optimal.

var tableUtility = (function () {
    var fn = {};
    // This is a constructor function for an object that represents a table
    fn.DataTable = function (p_data, p_columnDataFields, p_tableId, p_displayCallback) {
        // This is expected to be an array of objects.
        var data = p_data;
        // This is expected to be an array of strings that represents the names of the fields to use for the column data.
        var columnDataFields = p_columnDataFields;
        // This is expected to be the HTML ID of the table in which the data will be displayed.
        // Note that this JavaScript will only affect the TBODY of the TABLE.  It is expected that the HTML already contains the THEAD with the column headers.
        var tableId = p_tableId;
        // This is expected to be either null or a function to execute after the table is displayed.
        var displayCallback = p_displayCallback;

        // Find the TBODY element of the TABLE and cache it
        var tbody = document.getElementById(tableId).getElementsByTagName('tbody')[0];

        // softData sorts the table's data.  fields is expected to be an array of strings with the names of the fields to sort by.  sort functions is expected
        // to be an array of functions describing how to sort each field.
        // Note that fields must be distinct, or weird things will happen.
        this.sortData = function (sortFields, sortFunctions) {
            if (sortFields.length != sortFunctions.length)
                throw new Error('fields and sortFunctions don\'t have the same length.');

            var builtInSorts = this.sortData.sorts;

            var sortFunction = function (x, y) {
                var fieldIndex = 0;
                for (var i = 0; i < sortFields.length; i++) {
                    var first = x[sortFields[i]];
                    var second = y[sortFields[i]];
                    var sortFunction = sortFunctions[i];

                    // This allows a calling function to reference a built-in sorting function.                    
                    if (typeof sortFunction === 'string')
                        result = builtInSorts[sortFunction](first, second);
                    else
                        result = sortFunction(first, second);

                    //var result = sortFunction(first, second);
                    if (result !== 0)
                        return result; // difference found
                }
                return 0; // no difference was found over any of the fields                
            };

            data.sort(sortFunction);
        }; // end sortData

        // This object contains some built-in sorting functions that will be used on multiple pages
        this.sortData.sorts = {
            alphaSort: function (x, y) {
                if (x.toString().toLowerCase() < y.toString().toLowerCase()) return -1;
                else if (y.toString().toLowerCase() < x.toString().toLowerCase()) return 1;
                else return 0;
            },
            numericSort: function (x, y) {
                return parseInt(x, 10) - parseInt(y, 10);
            }     
        } // end sortData.sorts

        this.displayData = function () {
            var numberOfFields = columnDataFields.length;
            var numberOfRows = data.length;
            var html = '';
            for (x in data) {
                var entry = data[x];
                var row = '<tr>';
                for (var i = 0; i < numberOfFields; i++) {
                    row += '<td>' + entry[columnDataFields[i]] + '</td>'
                }
                row += '</tr>'
                html += row;
            }
            //tbody.innerHTML = html; I tried to do this without jQuery, but kept getting "Unknown Runtime Error in IE7"
            $(tbody).html(html);
            $(tbody)
                .find('tr:even').addClass('EvenBar')
                .find('tr:odd').addClass('OddBar');

            // If a callback function was set, invoke it.            
            if (typeof displayCallback === 'function')
                displayCallback();
        } // end displayData        
    } // end DataTable Constructor Function
    return fn;
})(); // end tableUtility