Data throttling when you purchase data

I live in wentworthpark, Krugersdorp and use cellc . when I have no data and use freebasics and only a WhatsApp bundle the network is super fast but as soon as I purchase data then the network becomes so slow that to upload a 5mb video on WhatsApp takes all day and while I is loading I cannot use the internet at the same time. I use a 4g Huawei y5 lite so it's not my phone .To me it seems as if cellc deliberately throttles the data to ensure I cannot use the full data allocation before it expires.