Turn processing times have been vastly reduced
Personally I have seen late game times go down from 1 minute before the patch to about 5 seconds after.

But also all of the bugs in my bug fix mod and most of the bugs fixed in WhatIsSols unofficial patch have been fixed plus more. The ones I can recall off the top of my head are:


The notorious MIRV bug where they were immune to PD and the warheads would spawn at the original ship instead of where the original missile disappeared.
Fixed the fact that the AI would never use leaders.
Made the AI be more careful when fighting monsters so they don't throw their fleets away over and over.
Fixed revolt lasting forever instead of 20 turns
Fixed leader buyout cost increasing cost instead of decreasing
Fixed Gnolam leader Arne providing a colony research bonus instead of an empire one (he is a ship leader).
Fixed Antarans sometimes spawning in the middle of the galaxy
Made traders and fantastic traders mutually exclusive so the traits don't conflict.
Made shapeshifters and telepathic mutually exclusive so the assimilation traits don't conflict (otherwise they cancel out).
Fixed Stealth Suit increasing missions risk instead of decreasing
Fixed radiated, toxic, and volcanic biomes having low enough pollution tolerance for you to be able to build up pollution and degrade the biome (the biome wouldn't actually degrade; the pollution just reset to 0). Set tolerance to 500.
Fixed dronespergroup in fighter bay tooltip on weapon screen.
Removed continuous modifier from laser cannon because laser cannon has 0 armor penetration. Now it's not wasted on AI designs.
Fix for Firing Race's Racial Trait - Beam Defense was being added to the defenders beam defense

Some new modding features were added as well:

Fixed changes to WeaponTypes.yaml not showing up in game
New galaxy ages and planetary density settings can be ordered using a parameter in those yamls
Biomes resulting from terraforming and pollution degradation can be modded in PlanetBiomeTypes.yaml
Fixed incorrect defense min globals in AntaranSettings.yaml


Oh and this one requires explaining from the patch notes: "Fix for incorrect population growth ratio." You see, in order to grow population, you need to reach a certain amount of surplus food. The forumla used to subtract bonuses, so if you needed 100 food originally, 25% growth would mean you only need 75 food, and 90% growth means you only need 10 food. If you do the math, that means that 90% growth actually gives you 1000% as much population. The formula has been changed, so now +25% growth means you need 100/1.25 = 80 food, and + 90% growth means 100/1.90=52.6 food. This means that 90% growth means 90% more population instead of 1000%. It was a huge problem before the patch.