The average speed of all the Pokemon in Alola isn't actually lower than other generations by much, the issue is the most popular/commonly used Pokemon are slow.
The faster ones are mostly rare and/or late or post game which is a deterrent for many players. Some of these include:
But then you have fast moving options like:
A lot of people would rather try out new Alolan Pokemon than use one they likely had before. This can also be countered if done properly, for example: whenever I battle a Bewear with Fluffy, I one shot it with my Salazzle using Flamethrower. If they placed A-Vulpix, A-Diglett and Minior on Melemele Island they'd get a lot less complaints about slow Pokemon. And you would have a more balanced hand. But many people ignore that.
Yet, with with proper balancing, for example, A-Forms like A-Raichu. There are plenty of Special Pokémon as well.
But you will notice that you mostly encounter Physical ones, whether it's because of Trainer representation, encounter rates or whatever else. They include stuff you can get like Abra. Or Magnemite. A lot of purely new Alolan Pokemon seem to be physical. Still, physical isn't the problem it's having moves that make direct contact that is the problem. Though to be fair finding a physical non-contact move that is SE on Bewear isn't easy but mons like Pelliper and other special guys are still available. It makes contact moves deal ineffective damage, and all fire moves are then super effective.
One example of how I play this is for me to use a 3/3 during my play through: Sylveon, Raichu, Rimbombee as specials and Decidueye, Golisopod, Bewear for physicals.