Exactly the same here - for 6 months (in Linksfield) I persistently got 30-40Meg, since 1st Feb, never more than 1Meg between 7 and around 11pm. I have a colleague in Randpark Ridge who is on a capped package as his tower / area isn't part of the Uncapped promotion and strangely, he has exactly the same experience!?
My call to Telkom didn't help alleviate any concerns I might have that this may be an ongoing issue - the lady that answered told me I just needed to remove my SIM can clean it and speeds would improve!?
Here's hoping there is some "light speed" at the end of this sad tunnel.
If it happened from the first then it almost certainly isn't high usage causing the problems. Must be Telkom messing with the network shaper. Hopefully they sort it out sooner rather than later.