We have multiple Apple compatible storage vendors competing in the TBolt3 SSD space: Sonnet Tech with it's Fusion Drive, LaCie with it's Bolt 3, OWC with it's ThunderBlade at the high end price points. However, NekTeck, Plugable and CableMatters offering blisteringly fast blade drives at the more affordable end of the spectrum using an OEM design they all share.
As with all solid-state storage, the Read speeds in the 2000+ MB/s tend to be more impressive than the Write speeds, but that's normal for SSD's in general. A lot depends on the files. Huge multi-gigabyte video files will transfer faster than a folder full of itty-bitty data files any day. But with the potential of unhampered 40Gbps Thunderbolt 3 throughput paired with a NVMe PCIe SSD module, you'll experience the most painless transfers or TimeMachine backups ever. If your wallet or needs are more modest, consider a modern 10Gbps USB 3.1 Gen2 external SSD drive with USB-C connector, it'll still be plenty fast, far cheaper than Thunderbolt-Anything, and still deliver snappy performance on a new Mac's Type-C ThunderBolt 3 port.