Chrome update of div problem

The code is a simple adding to a div’s scrolltop with this code

The div has several hundred k of html text, and when the above code is run, there’s a one second lag before it updates, as if the div is taking a long time to process.
However, if the same code is called from within a setInterval of maybe 250ms, after the one sec lag, updates then flash up instantly, indefinitely, showing that the div can be processed fine. I have read up about what causes reflows repaints etc but cannot work out how to get the instant effect without the lag in the timer or ideally outside the timer, its infuriating! thanks