That’s a genral symptom from transitioning from an old computer to a new computer. If the old computer was very old, probably all (including cheap) machines nowadays will be better and solve the problem (so one needs to take care to buy what is appropriate, and whether you really need a 1000 € graphics card).
Let’s get back to the same question
By rendering, what kind of rendering do you do, is it only the 3d viewport in SketchUp (OpenGL) that is not interactive enough, or do you do raytracing (rather CPU intense)?
What kind of models do you work with, how many polygons, is the model size maybe a signifiant reason for the slowness?
And finally think about what is your personal preference? Don’t force yourself using something only because of rational reasons, but let it be your own decision.
OS X is noticeably not little widespread in the domain of designers and advertisers (but the choice of operating system is also about what you feel and what you want to express). Graphics is a wide field, and for font design, layout, 2d raster graphics and photos OS X has definitely good applications. For OpenGL quality and performance, one can find however a bit controversial opinions.
You observe that you get mostly replies about OS X, but not about the other side resp. sides (since the world is multi-dimensional. That is because you are comparing a specific target (a small set of Apple hardware with Apple operating system) to a large, fuzzy set of the rest of the market. Certainly the average of the whole market is an inferior measure compared to something specifically good.
- Decide your use case and requirements first.
- Choose a small set of comparable computers from differend manufacturers that you could imagine to buy.
- Then compare and decide the platform question.