From perspective of Maya user, also, mental ray, and Renderman (Prman, 3delight) user... and of course Houdini User...
Mantra it's not slow!
this assumption that mantra it's slow it's become most people that use MR and Maya or Vray and Max, didn't count your render times seriously!
I mean, they count frame times, with no AOV, without sum average time per render layer, and or passes! and of course without Motion Blur!
If they do that i pretty sure that MR it's slowest render than mantra...
Assume that you need render a realistic car, if you only count the render time in render view of maya, you don't count the translation, also if you use a FG, you don't count the number of times that you must render until flickering is gone or free of artifacts!
If you put some passes and didn't use a rasterizer (with has a lot of issues of anti-alias), your time frame will reach the roof!
Not mention that for some heavy scenes, you will need separate in layers just because the memory blows away!
The solution? render AO in a render layer, render another layer with some shader to do motion blur in post etc...
if you take the time of all those passes (i.e render layers), and sum it, you will get a really slow render....
Ok, if you use a Vray/max you probably don't need or have a render layer, but you need to wait much more time to render without flickering! or not ?
I'm sure that was a tricks and workarounds... but it's not the same? or you will go to Kaos group forum and post something like:
"why Vray it's slowest render on the market?"
So, just because those people knows the issues about those renders, and use a workaround to produce your images, this is not means that it's not exist!
why instead they just come here and said, "too slow", they don't ask "what i'm doing wrong'?
Sorry for my english...
but i'm surround by maya and max users and all the time it's happens when people starting using a new software...
Also i'm still working with maya, just because there's no many places here in brazil to only work with Houdini, i hope that it's change, soon!
PS: using AOVs in mantra and Renderman, give you a full control of your passes, including per light, per objects and so, with a minimum cost in your render times....
Not mention too that RSL and VEX also gives you a freedom to make some impossibles things easier than programming a shader in C++ like others renders like MR and Vray...
cheers!