The Star Forge would be impressive in most universes, it can create essentially an endless supply of ships and other mechanical weapons and armor from the raw material in the star that powers it. To the Forerunners however, it is just a simply factory. Read this thread, http://forums.spacebattles.com/threads/forerunner-feat-thread.236388/ and pay special attention to the posts made by the user named Rama, he goes into extensive detail with mathematical proofs discussing the military and industrial might of the Forerunners. As an example, assuming the Onyx Dyson Sphere was constructed over a span of 8,000 years, "the Forerunners would have to assemble 1,120,716 cubic kilometers of material per second. To put it into accepted SW-vs-ST parlance, that's the equivalent of manufacturing the second Death Star every five minutes, non-stop, for nearly eight thousand years - or stripping away an Earth sized planet every nine days." This was a simple proof of concept project under a limited budget, and the Didact created at least 450 other smaller Dyson spheres under the same limited budget. The Forerunners strip mined entire planets--as in, the planets themselves were considered resources, and they broke them completely apart to harvest the resources deep in the core, as well as everything else. The Star Forge is frankly small potatoes, and I do not even think it is as fast the industrial capacity of a single Forerunner world. Forerunners could "grow" ships from a design seed within minutes, so if they grew one ship for each planet in their Empire at the same time, they would have 3,000,000+ starships capable of destroying a planet within the time it takes to go the gas station and fill your car (their empire held over 3million worlds, and the only design seed example we have seen created a 800 meter long ship that was classified as a "planet breaker" within minutes; imagine the Normandy from Mass Effect wasting a planet, and that was what one of the smallest Forerunner ships were capable of). The problem is, do they have the crew to man those ships? Yes. Aside from the simple population density of having 3million worlds at their disposal, they also had a heavy focus on hyper intelligent and advanced AI, some capable of commanding millions of ships simultaneously with no crew. So that's the gist of it, read that thread if you so wish for more details, it's fascinating.