onBeforeTimeHeaderRender is being run for every event when using gantt

The extra .update() when clicking the button is unnecessary I guess.
Does each events.add call re-render all timeheaders ?

The answer is related to you side question: "How do I add events in the fastest way ?"

The fastest way is to use .events.list and .update():

dps.events.list = [{...}, {...}];
dps.update();

The .events.add() and related methods (.events.remove() and .events.update()) add the event to .events.list and call .update() internally.

Every .update() refreshes the scheduler and invokes the rendering events (onBeforeEventRender, onBeforeCellRender, onBeforeResHeader, onBeforeTimeHeaderRender).

See also:
http://doc.daypilot.org/scheduler/event-loading/

I have tried now to manipulate the dps.events.list instead. But I get an exception.
Uncaught date parameter is not a Date object: function ($0M){if(typeof $0M==='undefined'){return new DayPilot.Date(e.data.start);}else{this.temp().start=new DayPilot.Date($0M);}}

In the example in the documentation you send in the date as a string for some reason, I assume it doesnt Have to be a string. Tried string anyway just to test but same problem. Can you see what is wrong ?

Here is the updated fiddle:
http://jsfiddle.net/Viktor/Yja42/5/

oooh I see now, the list doesn't contain daypilot events but some arbitrary object instead.