A 10 speed horse should be almost twice as fast as a 6 speed horse, right
No.
We can assign 10 to anyone 6'4'' or taller.
9 to someone 6'2'' to 6'4''
8 to someone 6 feet to 6'2''
Etc.
You can have a value that is never twice the lowest value but that has a grade that is twice the value. This has nothing to do with the actual thing being measured though.
You could as well assign letters- A, B, C etc. Is A twice as fast as F? No. It's just a letter and we understand that it is a higher grade than F, that's all.
I don't think speed is important.
I think stamina is though. I like to swim the horses. It's something that the horses can do and the ones with higher stamina swim further. It's something I do for enjoyment in the game.
I am ECSTATIC if you do not like to swim the animals. That's you and I am so happy for you. But I like to swim them. So, stamina is a relevant stat to me.
I disagree with one of his videos straight away. I've got some horses to do things others would not in the bravery department. He says if a predator roars, this is determining if your horse dumps you or not.
In my experience this is not strictly true. And I have got 2 level horses to stand next to the cougar- so bonding level isn't the sole factor here either.
But, hey. Your guy is a professional. So, I would advise you to put your stock in him.