Manual rendering vs. exporting mp4 files or else?

If the issue is that the animation is too complex for smooth playback in Motion, you can create a RAM Preview, which is akin to a rendered version in FCP.

Mark->RAM Preview->Play Range

or

Mark->RAM Preview->All

It may also be worth "pre-rendering" parts of your composition as well. For example, say you have a background made of composted elements that don't need to be modified (say a generator with some complex particles or replicators driven by behaviors). You could select the layers and use "Export Selection to Movie..." Export these pieces out to a movie, then disable the layers in Motion. Now reimport your rendered elements and keep working. Motion won't have to calculate the complex parts, it just has to draw the movie frames. This is an old compositing trick to make anything run faster, pre-render and replace.

One more thought, since we don't know how you'r project is setup, you can also disable more render intensive thing for better speed. If you have them turned on, turn off things like lighting, shadows, reflections, depth of field or motion blur until you REALLY need them. You should be able to work out your animation (design, pacing) without these and just utilize them for the final look.