I am a big fan of the Synology NAS. I am using this for almost 8 years and upgraded over time from my former DS210j two bay NAS to currently the eight bay DS1815+.
I am using the NAS mainly for iSCSI storage for my home lab. Of course I am also using other packages like surveillance station, download station, cloud backup, Sickbeard, Plex and others. To increase performance, I added two SSD drives.
Initially I wanted to add only one SSD, hence I have only 8 bays, but then the SSD will function only as a read cache. To use read-write cache, you will need at least two SSD drives.
I understand the why in an enterprise environment, but usage for usage in a home lab it is overkill. I needed read-write cache so at the end of the day I ended up with two SSD drives.