I don't think speed has been determined, since it has never run on Intel chips.
But on the Amiga's Motorola Chips, it was one of the fastest true 'Ray Tracers'

I don't think Impulse would port it over and not take speed into consideration.

In terms of features, and learning curve... ALL that you stated for 3DS is also
true for Imagine, and lots more... But I'll have to admit that after 3 years of
use on the Amiga, the learning curve is very steep. This is due ONLY to the
manual. It is realy BAD. However, there is a lot of after market support for
this product, including regular 'Tips' articles in many magazines such as "AVID
and a great book by Steve Worley called "Understanding Imagine 2.0" This book i
is not just recommened, IT IS A MUST!