It's about the primary gameplay loop, the fundamental mechanics that dictate how Sonic moves, at least as far as the complaints about the boost and Sonic's general movement go. From Sonic 1 to Sonic Adventure 2 (and Mania after the boost games), Sonic was built on gameplay of building up momentum, and rolling up and down sloped surfaces that would decrease, and increase your momentum respectively. I could go into a lot of detail about this, but suffice to say, it gave the player nuanced control and freedom of movement, with a satisfying, responsive weight to the character, and gave the environment an increased importance, since ramps and slopes are how you gained and manipulated your speed. It was fast and snappy, but flowing and freeing.

The reason a lot of people find the boost so contentious is that it takes away these nuances of control and game feel, of how it physically feels to move Sonic around in space, and the gameplay utility of that movement. Launching yourself off of stuff and figuring out how to position and time your jumps and rolling for maximum speed and height was a huge part of the draw of Sonic (and really, still is, hence why he's always struggling to keep his head above water, even with stuff like Gens and Unleashed).

As for why the slowness is a problem in Frontiers, besides the above, this is an open world, getting from place to place is already gonna be tedious with the more rigid, less nuanced movement of the boost games, making Sonic run at the speed of a bowling ball uphill is just gonna make it even more tedious, besides which this is a Sonic game. Even with gimped controls Sonic should AT LEAST be fast.