Minecraft digging speed attribute
Sadly that is not a thing however, you could use the efficiency enchantment like so:
execute at @p run setblock ~ ~-1 ~ dropper{Items:[{Slot:4b,id:"minecraft:stone_pickaxe",Count:1b,tag:{Enchantments:[{id:"minecraft:efficiency",lvl:200}],Unbreakable:1,display:{Name:'[{"text":"test pick","italic":false}]]'}}}]} replace
It will put a pickaxe with efficiency level 200 in the dropper