If you use large terrains and a graph with a lot of nodes, Instant Terra may require a substantial amount of memory to be able to compute everything.
If you count the number of nodes inside the graph plus the nodes inside component nodes and nodes inside component nodes that are themselves inside other component nodes, you have more than 300 nodes.
For 16K x 16K terrains, Instant Terra may require 64 GB of RAM or more.
One possible solution is to split your project into several smaller projects.
One project can export a terrain and another project can import it.
Or several projects can generate different parts of the terrain and another project can do the final compositing.
If you use Instant Terra Pro, you can also add some "Save point" nodes in your graph. Each "Save point" node stores its input in a cache on hard drive (SSD recommanded for better performance). When you open your project, if you don't modify the nodes before the "Save point" node, the "Save point" node will read the terrain from the cache instead of computing the nodes before it.