We've found that when the full-width treeview has many direct children under the parent record, there are performance issues - I think it is trying to load the entire tree into memory. Consequently, we've implemented a behavior to prevent all the system memory from being exhausted at once in this case, as described in the documentation here:
From the docs:
If there are more than 50 immediate children to the level being viewed AtoM will add the number of remaining records not immediately displayed in the top right corner.
Clicking on that number will load the next 50 records. If there are still more records not displayed, the number of remaining records will be shown and the user can choose to see the next set, or return to the beginning by choosing “reset”.
It turns out that there was a bug in this feature in 2.5.x. This should be corrected in the 2.5.2 release.
Unfortunately, at this time the behavior is not configurable. Note as well that this behavior is only when there are more than 50 immediate children beneath the parent record - in a multi-hierarchical tree (i.e. with many intermediary levels such as series, subseries, etc), the tree can load progressively, and this behavior is not needed.