Converting single Elements into meshes might speed up things extremely. The whole rendering process (and I don't know any renderer working on another base) is based on (flat) polygons, if you have a mesh it contains of nothing but polygons, so you don't need any additional calculation to create them. Bspline curves are quite the opposite, you need a heavy calculation to get the polys from and you (normally) do not know in which number of polygons they fall.
Converting elements into meshes in a single step give you more control. I.e. if you have some small rounded elements in dyour drawing, you need to set your polygon calculation tolerance to a very small value (i.e. 1mm). A single large sphere rendered with that small value might result in some hundreds of thousands of polygons (triangles). If you convert this single sphere with lets say a '1cm' tolerance instead of a '1mm' the resulting polygons are under 10% and the calcuation speed up.
The con against pre-converted meshes is, that you cannot work on them in the same manner, as if you still have a solid or surface and you can not use them to get closer to a pre-existing curve (you still have no knowledge about it) . So the question is, can I reduce a very large mesh with a preconversion or a simplification (as in your case) while keeping the 'roundness' of my objects as close to the original as possible. In such cases meshes are worth a try and the best base for renderings.