You don't want a lower resolution proxy file, you want to either use your camera original or tanscode to a DI (digital intermediate) using a good production codec. Proxy files are useless for real time editing on a NLE unless your system is below standards. Proxy files are only useful in AE if you are dealing with 4K or higher renders unless your system is right at the minimum system requirements. You'll want an external drive to store your renders and possibly your footage. Working at any resolution other than full HD is not a good idea because delivery to the web is all HD now, DVD's tanscode very well from HD material, and, if you reduce the size of your DSLR footage to SD widescreen you'll severely limit the things you can do with the footage as far as masking, color correction, keying, and any special effects or graphics you want to add. Also, do a bunch of work in SD and then try and output HD and you're going to end up with a lot of extra work or a mess.
I see nothing in your setup that would in any way require intermediate files. If it is slow, at most you may eitehr have an issue with hardware-accelerated H.264 decoding or I/O performance, both of which can be rectified with driver updates and using different storage, respectively.
...and since this is an After Effects forum, I have to add this: I hope you're using something other than AE for video editing. AE is the world's worst video editing application.
The DNxHD codec is a free download from Avid for both Win and Mac. It's a honey, too.