Removal of old IE legacy cruft is slimming Spartan's disk and memory footprint when compared with IE. Advances in the Chakra engine are pushing performance ahead. We're rearchitecting our DOM, which is yielding perf and security wins too. We're planning an extension platform also, with top add-ons like ad block being a clear target.
There's a bit of a catch 22 with "get users" and "watch compatibility fix itself" as broken compatibility is often cited as a top reason for users to switch browsers. It's hard to grow users without investing in compatibility.
So what we're doing is defining our "blend" of investments. Right now we have a heavy amount of interop investments in our blend as we think that's important for users. Over time (months, not years) the major interop gaps will disappear and I expect we'll see a shift in that blend to increase investments in other areas.
(Jacob Rossi, Spartan platform engineering team)