Does a file take longer to be retrieved from a domain to a subdomain or from a subdomain to itself?

I have a website and its mobile version (m.mydomainname.com), the problem is that Im in a bit of a dilemma with the mobile website. t has dependencies files like the javascripts, phps, etc. Im not sure if I should get these files from the main domain website or from their own folder. I do know that it would be easier to update them if they are only located in one folder, but I dont know if it will be reducing its speed or not. So the question basically is will it reduce its speed or not?