External SSD or HD. Performance compare to internal SSD?
An external HDD is going to be ~100 MB/s, an external SSD closer to ~500MB/s, the SD card expanders (such as tardisk or transcend jetdrive) have read/write speeds only slightly slower than that of the HDD (~75MB/s). However, the SD expander has the advantage of not actually being external (it sits flush to the edge of the MBP and occupies the SD card spot). Your internal SSD likely has speeds close to ~700MB/s. Network attached storage will only be as fast as your connection. If you have an "N" network, your speed will max out at ~35MB/s, with an "AC" connection or gigabit Ethernet you should have no problem getting full ~100MB/s speeds.