For now, GPGPU makes a large contribution, in a restricted range of cases.
As I understand it, GPGPU is enabled for all SQL Tile* functions which do not depend on neighbouring pixels. Whereas functions which operate over a window (functions using a border) are currently CPU-only.
For those SQL functions which are already GPGPU enabled, they are enabled everywhere in SQL.
In both respects, this is very different from the situation in Manifold 8.
And in the further respect that Manifold 8 did not use multiple CPU cores, while Manifold 9 does, whenever that makes sense and with minimal effort.
Bang-for-buck for now is to invest in many CPU cores, supplemented by a modest recent CUDA GPU.