If you've got lots of spare RAM, chances are the file will exist in the disk cache and won't actually be committed to the physical disk anyway. Windows already does a far better job at caching than you'll get from a RAMdisk.
When you flush a file to a RAM disk, it is definitely in memory and possibly on disk. When you flush a file to a real disk, it lives definitely on disk and possibly in memory.
If you think you're smarter than Windows (and that's not unlikely because Windows doesn't have a crystal ball to predict what you're going to want to do next), it's possible you might be able to use this to your advantage, because you have temporary files that live only possibly on disk, instead of definitely on disk, and thus avoid the IO-overhead of doing the flushes to disk.
I don't disagree with you that for most people the default of letting Windows get on with its job is probably your best course of action, I just disagree with your suggestion that RAM disks always slow down what you're trying to achieve.