One of the lesser known performance hindrances on an ultrabook is the low-power OPI GT2 mode on Skylake/Kaby Lake U/Y processors. If the manufacturer sets its laptop in this GT2 mode, all PCI Express components are capped at ~1,800MB/s. This means even if your laptop's Thunderbolt 3 connection has full x4 lanes, the bandwidth is limited. My guess is this was done to maximize battery life.
High-power OPI GT4 mode is what we want to make the most of a Thunderbolt 3 eGPU. GT4 mode will also allow a system to maximize the performance of NVMe PCIe drives. I read the 2016 version of the Razer Blade Stealth was configured in GT2 so I wanted to see if this situation has changed in the 2017 version (13" display on my RBS). I'm glad to report it has. Here's the HWiNFO64 which shows Razer configured both the Thunderbolt 3 connection and PCIe drive to use x4 lanes.
I didn't save a screen capture of ATTO speed test but I saw the read speed exceeded 2,200MB/s so it's definitely in GT4 mode. If a system is in GT2 mode, a fast NVMe such as Samsung 960 EVO cannot exceed ~1,700MB/s in ATTO. This Razer Blade Stealth is a very nice machine and one of the few cons most reviewers mentioned is the shorter battery life. With ultrabooks we find it a balancing act with regards to performance vs battery life.
While I was at it, I installed an RX 580 to provide a quick overview of performance difference between these two AMD cards. All benchmarks were through the internal display. You may notice the low Memory Write in the AIDA screen cap for RX Vega. I believe this is due to the dynamic firmware of the Mantiz Venus. When I ran that benchmark, I had an SSD connected.