DashboardGridFieldPanel::getHierarchy() is very slow

[purplespider]

I have a site with over 1000 pages. Dashboard was taking over 30 seconds to load.

Narrowed it down to DashboardGridFieldPanel::getHierarchy() which seems to take 10 seconds for each DashboardGridFieldPanel. This function simply populates the Page dropdown in the panel's settings.

So could do with optimising, or at least stopping it running on each DashboardGridFieldPanel load, as it is only required when entering settings for a single DashboardGridFieldPanel.