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.
At best, it'll be the same - letting Windows handle it will cache it or it'll be in the RAMdisk. At worse it'll be a lot slower, aside from the needless copying there is also the possibilty that the increased RAM pressure will mean that Windows will actually page out parts of your file and will therefore be swapping to and from physical storage to access it despite the fact you have a copy sat around in memory doing nothing, along with a whole bunch of additional free memory acting as RAMdisk free space.
If you push really hard you might find a extremely obscure edge case in which a RAMdisk beats the cache, but it's exceptionally hard to do. It exists purely for compatibility with diskless systems and it no longer makes sense to use it otherwise.