I don't think it quite works that way; it's really more about which information the OS keeps in memory at one time, regardless of how big the OS footprint is.
The thing about the slowness in the OS right now, is once you're inside an app nothing is bad. Speed inside individual apps ranges from decent/expected to really great (the browser). So it's not like the hardware is chugging trying to run the actual OS.
Rather, the "slowness" is in switching apps. Waiting for an app to close and get back to the dashboard and warawara. Waiting for certain seemingly small apps to open and initialize. Based on my understanding, that stuff seems like a combination of two things:
1. Unnecessary flushing of memory for the dashboard stuff given how much memory is available.
2. Certain applications are less optimized than others, and are bloated.
If this update actually is about improving speed, I'd assume they're re-writing parts of bloated apps and adjusting the OS's loading scheme to keep more necessary assets in ram while the dashboard is pushed into the background. Given how late they were in getting much of the OS ready for launch, I wouldn't be surprised if the day 1 update contained an old, but usable build of the OS and core apps. While a newer build of them was still under development.