If you download like mad then its fair to assume MWeb is throttling you but I dont have a lot of experiance there, only 2'nd hand knowledge, however if you would like to test for yourself you could try the below.
Go to Web Africa and/or Afrihost, they both have free 1 gig accounts that are great to use for testing purposes. If those work fine then its mweb causing you problems.
I've just run another speedtest and this time the download speed starts at ~4mb/s and then quickly decreases until it reaches 0.3mb/s and then still further decreases which basically freezes the test. Anyone had this happen to them?
Edit: Also thought I'd mention that my upload speed is ~0.4mb/s which is higher than my download speed