Network speed is measured by amount of data divided by time. Picking the wrong time frame can give illusion of higher speed. think about packets of size 2kb coming every 2 second, obviously, your "average speed" is 1kbps however, if your measurement window is 1s, then you get 2, 0, 2, 0, ... for your speed, which once averaged, gives you 1kbps again. The reality though is more complicated since packets are not received regularly and the PHY layer may need to go back and forth if a corrupted packet is received, so there is a lot going on before you can measure your application layer data rate (e.g. download rate).
It should not have to do much with your corrupted download, that is, they might have the same root cause or not.