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2015 Apple MacBook: SSD vs 2015 MacBook Pro Retina
Related: 2015 MacBook Pro, git, laptop, MacBook, MacBook Pro, Macs, source code and versioning software, SSD, USB, USB-C
B&H Photo loaned MPG the latest top-end MacBook with 1.3 GHz processor, 8GB memory, 512GB PCIe flash drive along with a USB-C to USB adapter (even better: Apple USB-C Digital AV Multiport Adapter and most users will want both).
√ Get the OWC USB-C Dock for MacBook.
As the graph shows, the SSD/flash drive in the 2015 Macbook is a strong performer in terms of 2014 technology, but as of 2015 the 2015 MacBook Pro Retina sets a new and very high bar to match, running its SSD up to 2.3X faster for reads and and 3.3X faster for writes.
The difference can be felt for common operations; your author uses the git source code control system and what might take 5-10 seconds on the MacBook Pro takes an interminable several minutes on the MacBook, suggesting that more than sustained transfer speed is involved (e.g., latency and transaction rates).
For ordinary use as intended for the MacBook (email, web, light duty), this difference is immaterial since programs are presumably left open and do not have to shovel around much data
The use of any external SSD will almost certainly greatly underperform the internal SSD/flash drive, thus arguing strongly in favor of going with the 512GB flash drive, unless storage capacity needs are modest. But photographers working in the field may find themselves rather unhappy with a 256GB flash drive, since OS X will eat about 70GB of that space, and about 20GB should be left free, so 246GB is really about 180GB usable space (note that Apple continues its fraudulent specifications for years now: 512GB is really 500GB usable, 256GB is really 250GB usable).