Vodacom Fibre line speed slow

We have been customers of Vodacom Fibre for a few years now - signing up with a promotion when they came into our area. Recently I was assisted by Vodacom with a router connectivity issue on the network that has now been resolved. As part of the problem-solving (the support was great) it was suspected that connectivity may have been affected by the FUP. However, this hasn't affected me before and we are on a 20Mbps line. Since the connectivity has been resolved, the line was initially very fast. Now after 2 or 3 days, if I run a speed test, it starts off at about 20Mbps but then quickly flattens out at about 4Mbps. Why is this happening now when it hasn't before?