Visually it is the same as the execution time implemented for the
geometry nodes, and it is to be enabled in the overlay popover.
The implementation is separate from the geometry nodes, as it is
not easy or practical to re-use the geometry nodes implementation.
The execution time is stored in a run-time hash, indexed by a node
instance key. This is similar to the storage of the mode preview
images.
The time is only implemented for full-frame compositor, as for the
tiled compositor it could be tricky to calculate reliable time for
pixel processing nodes which process one pixel at a time.
Sergey Sharybin