That's great your son is learning this. Are they teaching this in school now? I admit I have no clue.
All ISPs use the download speed in their marketing materials. Usually with the magic words "up to". So basically anything less than what they claim.
Maybe you can ask your son what is the best way to test the speed/quality. I think it has been mentioned before about the unreliability of speedtest as ISPs will give extra bandwidth to the site to make it appear that you are getting good speeds. Is that true? There was another site which is supposed to be better. Can't remember the name now.
If downloads are fast within Indonesia, but slow to connect to servers outside of Indonesia, what is causing this? In simplistic terms, I would imagine it is Telkom's own 'gateway' to the cable which connects the internet to other countries. You just need to look at a map like this and see there is not a lot of connectivity to Indonesia:
Submarine Cable Map