Vodacom steel our data

As you have Data on your phone it is going slow, but the moment when you buy EXTRA Data it is GOING fast WHY??? I bought 1 gig and in 12 hours it is gone, that is bull****, I am due for an upgrade but I will not do it, I will cancel my Contract. Now the call center want to tell me it is the way how I use my internet, I tell you it is a lie. I am on internet everyday and never check videos and as I use 1 GIG a day I will go to an cheaper service provider, then I must get 30 GIG a month. Never but never I use all my Data but the pass few Month my Data disappear like hell. Something is not wright at Vodacom