I'm not really sure if you like Sonic games or if you don't because your post is unclear to me, but I'll still quote you, so don't take it personally.
Because you don't like a game or a genre in particular, because you don't understand why some people have fun with them doesn't make them bad per se. I don't like rogue lite/like, but I don't go around screaming "Rogue games are generally not very good". It's the same for point & click, battle royale, MOBAs or whatever, I just don't like them, that's all.
As to how enjoy Sonic games... You can finish them in one go and don't have to bother with replaying them if you don't want to. Although, by doing so you'll miss a lot of what they have to offer. For example, one staple of the Sonic games is Super Sonic and if you want to play with him, you'll have to replay them enough time to find all the emeralds; and most of the time, the "true end" also requires you to have all the emeralds.
So no, you don't have to "play fast/memorize levels", but it is encouraged. More so in the 3D games because you have a notation at the end of each stages, like F, D, C, B, A and S, and those can be a requirement for unlocking stuff. As for the 2D games, like Mania, memorizing the levels will help you find the emeralds, but there's no point in going fast, except for one trophy/achievement that take place in Stardust Speedway I think, so you can take your time especially that now there's an option to disable the timer (I know a lot of people died in the last stage because of the timer).
EDIt: also this: