A small patch, and all of it is speed. The world steps faster at every speed you can set, big cities no longer hitch as you glide in toward them from the far map, and the card that names an animal now lands on the animal you can see. Nothing about the world itself changed — the same seed and the same deeds rebuild exactly the same history, down to the byte, so your saved worlds open exactly as they did. All of it is in the demo as well.
The world steps faster. The years go by roughly 1.6 to 1.9 times quicker at the higher speeds, and it is the same world, byte for byte. Two of the heaviest things a step did were doing work they did not need to: the water, heat and weather were carried across the map in a way that had to be redone whenever the work was divided up differently, and every single tick re-asked every town about a standing request that had not changed since the last time it was asked. Both were rebuilt. A step now costs between 39% and 47% less time, and the busier and bigger the world, the more of that you get back.
Big cities no longer hitch as you glide in from the far map. The far-map city silhouettes were rebuilt from scratch on every world refresh — at the busiest towns that was a stall of tens of milliseconds on every approach, and at the very largest a hang of most of a second. They are now built once per town, and updated only when that town's ground actually changes.
Hovering the world over open country no longer costs frames, and the animal card lands on the animal you see. Naming the herd, yard animal or bird under the pointer used to recompute every animal in the world each frame, and could name where an animal was about to be rather than where it was drawn. The card now reads the very positions on screen, including while a herd walks toward somewhere you have sent it.
About your saved worlds. Nothing to do, and nothing to read. The world's rules did not change in this patch, so a world you saved under v0.49.0 opens exactly as it did before: no retelling from its own history, no notice, and no slower first load.