A SSD for Roon’s database will speed up the overall performance. I use an SSD which is connected by USB to my QNAP NAS. I have not looked into Thunderbolt SSDs yet. (My model is not equipped with it). The SSD does not need to be very big. My ssd is just 128 GB and only 3% is used.
PS: Don’t mix up a ssd with a usb thumb drive… Just bringing this up, as other users got confused about it. 