Internet throttling

Every time I use free basics the network is super fast. But as soon as I BUY data the network completely stops working .It becomes slow and sluggish and a simple 5mb video on whatsapp takes all night to upload. Cell c is losing the plot very seriously. OR God forbid they are deliberately throttling data so we cannot use our full purchased quota before it expires . I live in wentworthpark krugersdorp in Rory street. Signal is poor at the best of times but when you Buy Data it just ceases to exist .