Yes, Capture instances geometry. But the question is more complicated - "I'm glad you asked!"..

Since you're looking to instance in another software and bring that in to Capture you may need to pay attention to which transfer format you use and whether that is able to preserve the instancing. It should be easy to check whether that works all the way through to Capture - compare an import of many objects with an import of twice as many objects and see how much the Capture project file grows. Keep in mind that each instance will be an object in Capture with some overhead, so if the geometry is smaller than the overhead it will be tricky to judge what happens - make sure you have a fair amount of polygons in the instance.

The next issue is instances by themselves. While the GPU loves them, the CPU can get quite annoyed if you have too many. It all comes down to the object count overhead and how each object is treated as a "shadowing unit". Something that can really bring Capture down is if you have too many objects..

Which brings me to the shadow bit, since you bring that up specifically. The fill rate of GPUs these days is so high that it's questionable whether the GPU will actually render a large amount of instances faster than a prepared "straight run" buffer. Chances are that transferring instance data to the GPU is actually slower than just having it render one large instance.

TL;DR - try both options and see which is faster on your hardware.

Thanks Lars, does Capture create instances when using the duplicate command (Ctrl-D)?