Howdy,
I had a similar situation with my Retina MacBook Pro recently (ran out of SSD space), and wanted to add more. I did not want to add an external drive that I would have to plug into it and tote around, so I opted in a "custom" 128GB Transcend JetDrive Lite (
http://www.transcend-info.com/apple/jetdrivelite/) card (which is custom MicroSD card shaped to sit flush with the rMBP). The company insists that it is "flash storage" as opposed to an SD Card, but it uses the SD Card controller in the rMBP, which leads to why I am replying to this post. The read speed on SD cards can be "up to 90MBs per second" with a write speed between 15 to 50 MBs per second. This really isn't as big as a problem as you would think, when you realize that a standard Hard Drive has write speeds (sustained not burst) of around 60MBs. Granted this is slower, but as a secondary storage it shouldn't be too bad. However, despite what logic would indicate, the JetDrive Lite does cause performance issues, but I don't think it has anything to do with the read/write speeds. It is my opinion that it is due to the limited number of simultaneous read/write cycles that can be done on an SD card. I don't know what the number is, but I am thinking it is pretty low. When only a single device is accessing the JetDrive, performance is Ok, but as soon as I start tying to do multiple things it slows to a crawl, freezing and hiccupping. If you want to go the SD Card route, I would suggest that you use it for storage only, like move your pictures and/or music to it. Perhaps even "archive" some of your documents folder there, but I would not suggest you try to install and run games to it. Personally, I added a 64 GB MicroSD card to my Surface Pro, and do just that, and put highly used or performance sensitive apps on the internal SSD. Good luck!
