My tree is just under 7k people, but have over 100k source citations with media.
I have over 100 people invited to online tree who add photos, stories, new citation links, etc.
I used FTM2012 and have compressed, synced, cleaned up locations, and cleaned media and sources several times and compress and sync frequently. The tree bogs down and locks up constantly. I have had to re-download tree from Ancestry several times because it loses the link. Each time, I have to clean up the media again because wrong media item is attached to citations.
There was a feature on Ancestry.com about a year ago that allowed user to link to a person in another tree, but required duplicate data entry in each tree. Now the link feature is gone. I am actually using the Weblinks to connect individuals who appear in multiple trees, but again having to do duplicate data entry in each tree.