I have an iMac (27-inch, late 2012). This is my plan: Replace the 1 TB hard drive with a 256 GB solid state drive. I will not store any user files on the internal solid state drive. My user files will be on an external hard drive. I'm thinking thunderbolt or USB3. My question is simply, will I take a performance hit? Would an external Thunderbolt/USB3 hard drive be slower than my original internal hard drive?