Without knowing the application, and exactly what's defines "super slow" it's hard to anyone (including VMware) to say why you're seeing a difference in performance.

 Fusion doesn't implement OpenGL for 3D acceleration - only DirectX 11 at this time.  If your application can use OpenGL, it may be falling back to using software implementations when running on Fusion - which could explain slowness.

If by the "other platform" you mean Parallels, they do support OpenGL.