You should have options and tools within Z-brush to reduce the exported mesh (poly-count)
Carrara has a “Decimator” in the vertex modeller,.. to reduce mesh,. but the issue is being generated in Z-Brush, ( by exporting a high poly model) then importing that into Carrara,.. you’ll get serious slow-downs.
the answer isn’t,,. “Fix this in Carrara”,. it’s,,. “Fix it in Z-brush” by reducing the definition of the High polygon surface you’ve created in Z-brush,
Export a Displacement or Normal map from your High definition model in Z-brush,. then reduce the models definition and export,..
In your 3D program,.. load the low / medium poly model,. then apply the high detail Displacement or Normal maps to make the lower poly model lok like the High poly model.
Hope it helps