I really don’t think using a million bits here is required. I think you could get an accurate picture of the BER by using considerably less. However, you can try removing many of the visualizers. Each visualizer will save the data of the signal, which will take up a lot of memory while the program is running. It will slow down the simulation considerably. Try removing all but the essential ones you need. Make sure that the data monitors are taken off too. They are the small rectangular icons on the output ports of components.You can use the Data Monitor tool on the right hand side of the layout to remove them. I have attached a picture of where they can be found.
The Matlab component at the end is causing an error for me, because there is no associated m-file.