Some of the jquery plugins shipped with 2.5 has been optimized for large sections (+100 fields) and also features and brand new jquery plugin: the affix plugin.
The optimization also brings new public events on the collapsible plugin:
updatesize.collapsiblewhich will invalidate the cached values for the height of the DOM elements.setsize.collapsiblewhich will set the min and max height CSS properties based on the cached values.
If you develop a field that uses the collapsible plugin, feel free to trigger those events.
The new plugin, affix will make a element "sticky" on the page, in order to "follow" the scroll. This plugin is used in the section editor in order to make the Expand/Collapse button always visible.