Here's the thing and the catch all the time... Your subscription is an internet service which is "up to 5Mbps" and not simply an internet service giving you an average of 5Mbps so anything in between 0 and 5 Mbps fall into your service. It goes without saying that Globe is true to their service and the loophole in these "up to xxxMbps" tags.
Of course you deserve more and the only recourse is to report it all the time until you get satisfied. If not, then it would be hard to build up your case and justify your reasons unless you elevate it to the NTC which fortunately is against practices (but seems powerless at this point) of promoting services this way.