Assuming the current rate of increase in processing power computers eventually will get to the point where they would be able to calculate a human mind (or non human mind for that matter). Initially computers wont be able to render them in real time. It may be the case that it could take something like two weeks to render 1 second of relative time in the simulation for something as complex as a human brain. Exploring what AI could do with this kind of processing power is very interesting but let's focus specifically on the emulation of human minds now and not the creation of artificial intelligence.
People who could possibly get their brains scanned might be early candidates that would be willing to live in a simulation that runs so slowly. Even though it would seem slow to the outside world, there would be two consolations. One is that relative to others in the simulation, time would flow normally. It might take many weeks or months for the simulacrum to have any kind of conversation with a human, but they would all be able to talk to each other at a normal rate so nothing would seem out of the ordinary.
The other would be the promise of faster computers. Computers keep increasing in speed year over year and there seems to be no end in sight. A lot of people talk about Moore's law hitting an atomic level, but I don't believe that will be an issue. We've already moved to multi cores. Just like there was once very few transistors on CPU compared to today, eventually there will be million or billions of CPU's in a single computer. We are network computers together and can use multiple computers to do computations on a single project.
As computer speeds increase eventually the simulacrum will be running in real time, able to have conversations with people in the real world. Eventually computers could be running so fast the simulacrum will be running faster than people in real life. Eventually it might be twice as fast, four times as fast, etc.
From the perspective of the simulacrum the real world will become slower and slower. Scientists in the simulated world will make more progress in theoretical physics, mathematics, biology, and other fields which can be relayed the real world. Perhaps giving them new ideas for material design or inventions that could be implemented in the real world to create faster processors to further speed up their world creating a feedback loop of innovation.
Experiments would still need to be done in the real world, at least until a grand unified theory of the universe is discovered and experiments in the simulation would be just like those done in real life as at that point it would simply be a matter of crunching numbers.
As the processing speed increases the virtual world would provide a lopsided contribution to society. Imagine if the virtual world were moving at 100 years for every day of real world time. There would be a steady high volume flow of scientific theories. Scientists in the real world wouldn't be able to keep up with all the revelations. Musicians, movie makes, novelists would still be working in the simulation but the real world would be getting music hits the like of which aren't heard but every 100 years on a daily basis. Imagine one day the full collection of Bach comes out, then Mozart, then The Beatles, then Michael Jackson. Everyday a new novel greater than any in history previously would come out.
Would people in the real world be in a constant state of amazement at all the art being produced or would their expectations increase so literature, film and music of the highest order is what is needed to keep them entertained at that point. I would guess the later would be the case as a game of marbles seemed to be adequate entertainment in the past and now video games provide much more stimulation. From the perspective inside the simulation the real world would slow down to a stand still. They would see people in the real world as something like mountains, they would know they move but on comparatively geological time scales. Even though in a certain sense the people in the real world would no longer become a factor in the interaction of people, their world would still be completely reliant on them as caretakers. Maintaining the computer systems, electrical grids, preventing asteroid impacts, etc.
As people die they would likely be scanned and added into the simulations so it would become a kind of after life. Because of the difference in speed, people entering the simulated world would be a much less common event than a child being born int he real world. If you have a fully functional simulation you could have children born to purely simulated entities, being an intelligent perfect replication of a human but having only ever existed in a simulation.
If you had simulation that was hybrid, there would be great benefits. Assuming a 100% accurate simulation that exactly mirrors physics in the real world, you would have the chance of injury, aging, etc. If you use the physics but augmented other systems, like daily backups of each entity, safeguards against injury where terminal velocity from a fall would be a not harmful impact on the ground. You could make modifications to DNA, try it out for a while, and revert back if it doesn't go well. Although simulacrums might initially have a world that closely resembles the world they are comfortable with, as time goes on I suspect more and more modifications would be implemented improving their reality. After hundreds of years it might be unrecognizable to the real world. Imagine houses like Tardises (For those of you unfamiliar with Dr Who, houses that are bigger on the inside) You could have an apartment building but behind each door could be miles of house, with mountain ranges, etc, all available to each resident of the apartment building.
Instead of playing first person shooters you could really be running down a map and if you die, you are simply re-instantiated at the beginning of the map with all your memories.