[inquisitor]

During the past two days I've been using an Orange prepaid SIM which I bought for € 9.90 including € 10 of credit over the past two days. After an additional top up of € 5 I was able to subscribe to their 1GB pack with a validity of 30 days for € 15.
Despite there is LTE800 (band 20 with 10MHz carrier bandwidth) coverage in the city of Luxembourg their LTE network is a total mess. Even with perfect signal strength throughput comes to total halt again and again. The best data rates I could achieve in the middle of the night were in the low single-digit Mbps range and I had to force my phone to 3G to get reliable Internet access throughout the day. Data rates on 3G weren't much better but at least without those disruptions.
I haven't tested the other networks, but Orange seems to suffer serious congestion problems on their 4G network and the 3G network is subpar as well.
I've tested the SIM in a Huawei E3372 USB modem as well as on my Samsung Galaxy S5 with the same results which rules out my device as a cause.