so i am getting very slow loading with a test data set of 50 000 (would require much more later on , but your performance demo seems to cope with those numbers.), I have inspected your performance and big data demo and copied some of the configuration in order to try and get better loading speeds but i did not have any success. here I have attached images testing parse (put the 50 000 in the html file) and load functions.
using gantt.load()
here is my code 
<!DOCTYPE html>
<head>
<meta http-equiv=“Content-type” content=“text/html; charset=utf-8”>
<script src=“https://cdn.dhtmlx.com/gantt/edge/dhtmlxgantt.js”></script>
<link href=“https://cdn.dhtmlx.com/gantt/edge/dhtmlxgantt.css" rel="stylesheet”>
<style type=“text/css”>
html, body{
height:100%;
padding:0px;
margin:0px;
overflow: hidden;
}
</style>
</head>
<body>
<div id=“gantt_here” style=‘width:100%; height:100%;’></div>
<script type=“text/javascript”>
//*
gantt.config.date_format = “%Y-%m-%d”;
gantt.config.smart_rendering = true;
gantt.config.smart_scales = true;
//*/
gantt.attachEvent(“onLoadStart”, function(){
gantt.message(“Loading…”);
});
gantt.attachEvent(“onLoadEnd”, function(){
gantt.message({text:“Loaded " + gantt.getTaskCount() + " tasks, " + gantt.getLinkCount() + " links”, expire:8*1000});
});
gantt.config.min_column_width = 30;
gantt.config.scale_sheight = 60;
gantt.config.date_scale = “%d”;
gantt.config.subscales = [
{unit:“month”, step:1, date:“%F, %Y” },
{unit:“year”, step:1, date:“%Y” }
];
gantt.config.row_height = 22;
gantt.config.static_background = true;
gantt.config.start_date = new Date(2018, 0, 1);
gantt.config.end_date = new Date(2019, 0, 1);
gantt.init(“gantt_here”);
gantt.load(“/data”)
</script>
</body>