The reason for the problem you are facing in YAM is, that YAM uses the NListtree.mcc class for rendering the folder list. This class is a subclass of the NList classes and highly depends on the internals of the NList.mcc class itself.

As a matter of fact, the NListtree class is known to be somewhat unoptimized in the fields of redrawing entries. Somehow it seems to massivly redraw items if there is any other window in front of the list it wants to redraw. So, this is a known issue of the NListtree class and will hopefully be adressed in one of the future versions of NListtree.mcc itself.

However, you can perfectly try to avoid that slow redrawing situation by making sure that no other window is in front of the folder listtree in YAM as soon as it requires redrawing.