Yeah you’ll get a better overall performance doing that, but once you’ve experienced the speed increase you’ll wish your samples were on there too lol, projects running from a SSD not such a performance boost in my experience compared to the OS and Sample Libraries going on SSD - depends on your audio track count of course.
Also, if you have any current issues with the Cubase install that may also help clear those too - are you putting High Sierra as a new install or cloning the drive? If you’ve not done a clean install for a while it may be a really good opportunity to lean your system out and go clean, especially as you have everything on the internal which you can copy across/refer to.