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>> How to speed up scrolling while item is dragged? 

You can make scrolling faster by modification in the dhtmlxtree.js. You can try to increase the step in the _autScroll method: currently this step is 20.

dhtmlXTreeObject.prototype._autoScroll=function(node,a1,a2){
  if (this.autoScroll)
  {
  if (node){
  a1=getAbsoluteTop(node);
  a2=getAbsoluteTop(this.allTree);
  }
  //scroll down
  if ( (a1-a2-parseInt(this.allTree.scrollTop))>(parseInt(this.allTree.offsetHeight)-50) )
  this.allTree.scrollTop=parseInt(this.allTree.scrollTop)+20;
  //scroll top
  if ( (a1-a2)<(parseInt(this.allTree.scrollTop)+30) )
  this.allTree.scrollTop=parseInt(this.allTree.scrollTop)-20;
  }
}

>> Is there any way to scroll to the top at once?

You can try to use focusItem(itemId) to set focus at the first item.

tree.focusItem(tree.getItemIdByIndex(rootId,0));