Zoom on drag, not after
Zooming involves pressing the command key and the space bar, and then clicking and dragging the mouse. The speed and duration of the drag is replicated in the zoom on screen. Or so it used to be.
All of a sudden now, when I click, instead it draws a box, the contents of which then fill my monitor — but only when I release it.
It used to zoom in on the mouse drag - but now it waits until I’ve completed the box. How do I toggle back to the previous settings?
The 'marquee' way of zooming is the way it worked before GPU rendering was introduced.
So it works only in GPU mode — check it you have enabled in View > GPU Preview.
Plus you should check if Animated Zoom option is enabled in Preferences > Performance section.