There is no reason for Docker to be slower, it must be some kind of configuration issue. Containers are basically just processes and there is virtually no difference in execution times.
As I said in my other reply, Docker is more likely to have ephemeral storage for the cache. So every single lock it'll re-download the package. Whereas locally, you're likely to still have the packages cached.
This can make a difference of tens of minutes for some packages which have a 1 gigabyte (!!!) download.
This can make a difference of tens of minutes for some packages which have a 1 gigabyte (!!!) download.