I have no idea about the speed comparison. I did, however, purchase an Axiom 9 a year ago to replace a 2004-era C80.Basically, I'm very happy with it. At the moment, it's totally un-networked, so it's just a plotter operating off of its own internal GPS. As such, it does 98% of what I need out of a plotter. I think the speed of the processor is just fine, actually much better than the C80. To zoom and move around, it's just like an iPhone or iPad, with pinch and one-finger moving. I'm finding that to zoom it in and out, it's faster and less error-prone to just use the big + and - soft-keys on the screen, as compared to pinching the touch screen, but the pinching gesture works fine. It's just a little bit fussy on a moving boat, compared to the +/i "keys".
I did buy a SeaTalk2 to SeaTalkNg converter to network in the older instruments. Haven't installed it yet. One great feature of the Axiom display, though, is that you can place data fields on the screen wherever you want them, such as SOG and Depth. Having a large SOG read-out right on the screen is something I really like, and Depth would be even more useful, since my depth-sounder display happens to be half hidden underneath the plotter (in a lower-level console). Really annoying when you're in thin water in a channel. So once I get the older instruments networked to the new plotter, having the depth right there on the screen will be huge. Not to mention waypoints and following a route with the autopilot.
Anyway, I can't advise about Axiom vs. Axiom+, but the Axiom seems to be plenty fast as far as I'm concerned. Good luck with your decision.