@EvilLucario if you look at the actual speed of Sonic in the game (using parts of the level as frame of reference), he’s considerably slowed down in most 3D games, especially the Adventure titles. (Try playing them and measuring out some distances then seeing how quickly he goes across them to see what I mean.) Sure, he’s moving through the stages much faster than Mario, but there is a huge difference in speed for him. The 3D ones he moves faster tend to be the ones with the camera fixed behind him at all times. Unleashed was honestly one of the best 3D titles at maintaining the speed... except more than 2/3 of the game is the Werehog stages, and then the Day stages end up being ruined by being forced to slow down to collect coins. I imagine the reason there is that, quite simply, when you DO manage to have Sonic move at the faster speeds (one’s comparable to the 2D games), a lot of stage has to be modeled only to be seen for seconds, making it expensive and time consuming... leading to, as you note, padding. Perhaps I should’ve worded it that part of the reason it doesn’t translate well is it becomes expensive to make a game of reasonable length when you do manage to have the character move at speeds similar to the 2D ones, and they have to fill it with other stuff to not make it a money sink. Especially given the expectations for a AAA game nowadays.

Even the Adventure games, which as I noted have Sonic moving at a much slower speed than most of his 2D titles, when they have levels where the focus is moving quickly... well, let’s take Final Rush as an example. The stage has essentially three types of areas: 1) areas with a lot of stuff that ultimately loop the player around a lot, reusing the space to give the appearance of speed without having to cover much ground, 2) areas that are longer and sparse in scenery, often with one or two grind rails to jump back and forth with and an enemy every so often, and 3) areas where Sonic is forced to slow down, whether to climb some vertical sections or deal with other obstacles. All of this is done because making a 3D level that a character zooms through, when done well, is a lot of resources. The more effort they put into those stages, the less game they can afford to make with them, requiring more padding. And sure, they could’ve cut the other characters and just had the Sonic/Shadow stages, but the length of the game would end up being too short that people wouldn’t wanna pay $50+ for it despite the budget being equal to other full retail titles, that or they’d have to compromise quality more.

So to make 3D games that capture the feeling of speed with Sonic and do it well and focus entirely on that gameplay? It’s not a very realistic request. Better to let the 2D titles focus on higher speeds. 3D doesn’t have to be all slow exploration, but it’s reasonable to have different expectations there because it’s much more expensive to produce a 3D game the player rushes through than a 2D one. And the cap speed does have to be lower because of how 3D information is processed. Adding an extra dimension adds a layer of complexity that can make a high speed game less approachable for new players (unless you give them less obstacles or spread them out more). Which is why some stick the camera behind Sonic at a fixed angle, letting players just focus on what’s in front of them. And that works but it’s not necessarily the best solution. Heck, camera becomes one of the biggest difficulties in a faster 3D game, either you fix the camera angle (which comes with a bunch of limitations) or let it move (which can make it an obstacle for the player).