Maya is Renderman compliant.

Not really. To say that something is Renderman  compliant, it should support several if not most of the features as outlined in RI spec. Out of the box, Maya doesn't / isn't. Curiously Poser is closer to the RI spec. It's just that over the years, the integration between Maya and PRman has grown, while Max and XSI are no longer supported.

...it's supposed to render pretty fast...

As mentioned, raytraced effects are Renderman For Maya's forte. But displacements are free (negligible impact on render time) and the motion blur is super duper fast and much cleaner than mental ray's. But to really make use of the speed gains (a) you might want to wait until the next version, which will support unlimited number of processors and (b) you must consider catering to the renderer every step of the way:

• keeping poly weight low either using NURBS or subDs. Calculating SSS on a Mil3 or later character will choke your system for hours.

• keep all textures square and convert to mipmaps

• limit raytracing as much as possibly by using reflection maps  and selecting only certain items to participate in raytraced reflections; and "deep shadows" which are semi-transparent depth mapped shadows

• pre-processing and caching ambient occlusion and SSS. The caustic IMHO aren't very good.

• pre-compiling shaders to avoid conversion during the scene translation. Renderman Pro Studio does this semi-automatically.

Is it worth all the extra work? Yes. It's pretty fast, the motion blur is stunning. Learning curve is steep.