Earlier this week, Nintendo rolled out the Wave 4 DLC for Mario Kart 8 Deluxe on the Switch. The patch notes specifically mentioned how Version 2.3.0 improved the performance of "some characters and vehicle customisations" and now we know what exactly has been updated.
Here's the rundown courtesy of the Japanese website Mario Kart Blog via Meester_Tweester on the Mario Kart subreddit) - explaining how most racers have had their ground, water, air and anti-gravity speed stats increased. Other characters, vehicles and tires have also had their mini-turbo values increased.
One other thing to note is that the "timer glitch" in Time Trials has been fixed, making recordings more accurate.
You can view the original patch notes story in the link below. Version 2.3.0 major content update as noted was the Wave 4 DLC - including 2 new cups, eight tracks (including Yoshi's Island), and the classic character Birdo, who originally appeared as a racer in Mario Kart: Double Dash!! on the GameCube.
Interestingly, there are now an additional five slots in the character selection menu, with Nintendo previously confirming more racers from past entries in the Mario Kart series would be returning.