PCMark and Windows File Transfer
PCMark's data drive benchmark examines NAS products through three traces that entail copying 339 JPEG files to the server (write test), making a copy of said files (read-write test) and then copying the files to another drive (read test). In this specific scenario, the DS920+ is deemed a healthy upgrade over an older, less powerful Synology box.
Curiously, this particular test actually suffers when SSD caching is enabled. With the 400GB SNV3400 M.2 drive installed as a read-only cache, bandwidth drops to 12.9MB/s while latency is pushed up to 1,657 microseconds.
Copying files and folders to and fro the NAS from a Windows 10 client reveals similar results. Copying a directory from the NAS was able to benefit from the SSD cache on subsequent runs, with average throughput increasing to 96.1MB/s on a second run. A modest bump, though do remember the target for the SSD cache is to improve IOPs as opposed to MB/s.