Is my unlimited data sim being throttled?

I use the unlimited data only sim in a tablet, using it as a hot spot to get access to the internet for my computer as I don't have a BT phone line.

But it is selectively very slow (128kb/s to 1.5mb/s) but when i go to a speed check website it happily reports an average 20mb/s .

Is smarty throttling my internet access or is it Three?

I see here on this forum it seems to be a common issue with Smarty but is put down to signal strength or the customers fault, but how does that explain a 20mb/s speed test and 128kb/s to 1.5mb/s while on Social Media, Video Streaming and my email?

Any help would be appreciated Smarty.