I'm not sure whether or not the copy is done immediately. There may be delayed writes but accesses should still be fast as the accesses should go to memory; not to disk.
On building products, the benefit to compiles would be to access header files that are commonly used. On links, to other object libraries. If you have a ton of memory, the best use for performance is to dedicate a chunk of memory to a RAM disk and then do the whole build off the RAM disk. I had a friend with a heavily loaded Mac Pro try this on a Firefox build and he reported some amazing times. I don't recall if it was a quad or octa core system.
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