My current location is thousands of kilometers from Ubuntu's main server and I am obliged to use one of its mirrors in my country of domicile.
Do the owners of Ubuntu implement measures to periodically check their files (eg. ISOs, updates, security patches) at their mirror sites have not been tampered and/or malware/trojans have not been introduced by hackers?
My personal experience with installing and updating Ubuntu from the main server took at least two hours whereas the same process took just 10 to 15 minutes when I used Ubuntu mirror site available in my country.