If you take a look at this thread, you’ll see that others are struggling to get Keyscape to run without buffer underrun as well - even on a pretty high-powered i7 machine with 32GB of RAM.
So it looks like Keyscape is truly a demanding resource-hog; maybe not the best instrument for live use? Maybe start out with some more resource-friendly plugins to build your setup and work your way up towards the more demanding ones step by step.
My favorite resource-friendly instruments as possible substitutes to Keyscape are:
- Addictive Keys (XLN Audio)
- Lounge Lizard (AAS)
- Pianoteq 6
These should cover a lot of similar ground - not at the same quality level, but definitely good enough for live use! I’ve got them at the core of my live setup (current pre-loaded setlist around 160 plugins!), and I use them with absolute confidence.
There is also a whole thread on resource-efficient instruments that lend themselves best to live use - look here.
Cheers,
Torsten