6 GB free space on your system drive is asking for disaster. I would treat this as a data emergency. Do not run Aperture. Back up your drive (preferably by cloning) and then trim it.
My casual observation is that Aperture uses at least part of the Library drive as a scratch drive. I have seen performance decline when the Library drive goes below about 10% free space. I aim to keep 20% free space on any drive that hold an Aperture Library. (I am just a user and not a hardware engineer or even a hardware maven.)
I purchased a mid-2012 15" MBP-Retina to run Aperture (2.6 GHz Intel Core i7, 16 GB RAM, 500 GB SSD). I am pleased with the purchase. This was a sooner-than-expected upgrade to my mid-2009 13" MBP (2.26 Core 2 Duo, 8 GB RAM, 500 GB HD). The upgrade was driven by my using an iPad-Retina. The display is unrivaled -- so much so that I do as much editing as possible on the MBP-R and less than ever on my large NEC monitors. (Imho, the Retina display is already completely changing our expectations of what pictures should look like. While shooting tethered for run-of-the-mill clients, I have had to move the Aperture Viewer to my 15" screen from my 27" NEC wide-gamut (PA271W) monitor at the client's request. "Is your big monitor fuzzy?" is not a question one wants to hear.)
I notice the difference in Aperture performance every time I use my old machine.
Search the forum for informed discussions about spec'ing a machine for Aperture. The Air is out of the question, imho. When I bought my MBP-R there was no option for upgrading RAM after purchase, so I went for the max of 16 GB.
My experience with performance is that I can't have enough. This machine has allowed me to do things -- extensive work with source Images 7,500 x 10,000 px -- that I wouldn't have tried before.
Budget for storage and back-up. I advise buying drives for Aperture in _triplicate_: one for the Library and two, rotated, for on-site and off-site back-up. (As LĂ©onie points out, the Library should be a _fast_ drive. Originals should be on "not-slow" drives. Back-ups can be on anything that works.) This has nothing to do with whether you make a living as a photographer: only you can place a value on your data. Your data, and the time you put into organizing and manipulating it, are irreplaceable.
My experience with USB-3 drives is that they work great. If yours works haltingly, something is not right.
HTH.
-Kirby.