"Also, how is the rendering speed of PDFs compared to that of !PDF?"
Rendering speed is pretty good (and they have announced that they are working to speed it up further). !PDF is slightly quicker than RiScript, but that's not comparing like with like: unlike RiScript, !PDF doesn't convert a document's fonts (which is a very important feature of RiScript); doesn't support clipping, which can make for messy images in !PDF; generally, !PDF allows you to read documents, but doesn't guarantee that everything is exactly as created.
Strengths of !PDF, in my opinion, are: F4 for search -- I *really* hope that that's on the wishlist for RiScript; the ability to open multiple windows. In true RISC OS tradition, I use both
Anyway, the new RiScript is, for me, a big step forward, with much-wanted features and the promise of further development. Very good news!