The thing is, internal storage isn't necessarily faster. Sure, it is on the top tier $750 and up premium-priced phones like the iPhone 6s (which has very fast internal storage). But we're talking about a
much less expensive phone here. Let's compare:
64GB Nexus 6P: $549
64GB iPhone 6S+: $849
So, you really shouldn't expect the Nexus 6P to be using the top-tier internal storage solutions Samsung rolls out as used in the iPhone and Samsung phones. Yes, a lot of that $300 price difference is for the Apple name, especially when you factor in the lower resolution 6S+ 1080p screen. But quite a bit of it is used for the polishing bits that make the phone faster... like much faster storage.
As a data point, the high-end Sandisk 64GB microSD cards are about $50 and are around 3x faster than the internal storage in my Nexus 6. They're hitting 95 MB/s read, 90 MB/s write now.
While a microSD won't top out at the 200 MB/s you get from last year's top-tier storage available in the Samsung S6 and iPhone 6 (not to mention the twice as fast Samsung storage in the new iPhone 6s and upcoming Samsung phone), they'll perform better than the internal storage in most mid-tier phones and all low-tier phones, which is all most folks need for photos and videos.