Both. This add-on will primarily create a collection of stone pieces to build the stone wall. Additionally, it also adds a suitable shader material for the perfect stone appearance. If you want to use your own stone material with the generated mesh objects, you can easily do that as well.

It can take several minutes to execute because this add-on deals with hundreds or thousands of objects, their geometry and shader. This is a normal behavior, just like similar add-ons e.g. Cell Fracture add-on. You can reduce this wait time by planning your job suitably. If you need to create a large wall or a large ground with stone, create a small part of that wall or ground. Then convert it to a stone wall and create several instances of that stone wall. Combine them together to form the large wall/ ground. It not only reduces the execution time, it will also reduce the load on your RAM since much of the data we are keeping in the memory is instanced data instead of real object data.

There are two options in the operator box: Decimate and Simplify. Both of them will reduce the vertex count resulting in a lower poly. You can turn on both, but don't use a very high decimate level because it can also destroy the shape of the stones. Using these options can further increase the execution time, so don't use them when you are experimenting with the look & feel of the stone wall. Enable them when it is final. Additionally, you can convert parts of the wall into instances as described above, it will also reduce the load on your machine.

No. This add-on will create only rectangular stone wall, or rectangular ground, pavement, road etc. If the selected object is not rectangular, the add-on will still work - it will create the stone structure based on the object's bounding box (which is always rectangular).