Aldi modem suddenly going slow- Germany

[inquisitor]

Probably because you are outside of 3G coverage (3G coverage is indicated by the modem blinking blue, otherwise it's green) or your local cell is just congested, like most of the eplus network, which Aldi uses.
If you want fast and reliable internet access at resonable rates get a Tchibo or Fonic SIM card. Tchibo currently sell their SIM at a promotional price of € 1.
For further information on Tchibo and Fonic refer to the Home - Prepaid Wireless Internet Access