I wouldn't really recommend using TG2's internal erosion operator. It works, but as you pointed out it is slow.
However, if you're patient you can just let it finish. In my experience it can sometimes freeze but mostly it finishes even though it looks frozen.
How to avoid you need to re-erode the terrain after you have restarted your PC/session/projectfile:
Generate the heightfield (or if you loaded one, load it in)
Erode the heightfield once.
Connect the heightfield output from the red node to a compute terrain.
Create a new "heightfield generate" operator.
Connect the compute terrain to the "shader input" port on the right of the heightfield generate node.
Inside the heightfield generate node go to the "Use shader" tab and see that the compute terrain is connected.
You'll see a couple of settings:
The coordinates are default 0,0 on X and Z. You can choose to let these coordinates be the bottom left or the centre of the square heightfield.
The size of the heightfield is 10x10km by default.
The number of points defines the resolution. Try the default settings and see if you need more points.
If everything is set then hit "generate" and the operator will create a heightfield based on the terrain provided by the compute terrain.
Right click on the node and choose "save as".
Now you have your island terrain file eroded and ready to be opened anytime you work on it.
Sounds complicated, but it really isn't.
I've attached a clip which shows the basic setup.
Of course you can also load the terrain file into a free version of World-Machine or Geocontrol and see if that works for you.
They might have limitations on resolution, so you need to check out for yourself which way is more feasible for you.
Cheers,
Martin
After you've eroded it you can save the heightfield
