You're quoting your speed in bits per second. Remember divide kbits by 8 to give you bytes per second. So really your download speed is only about 8 KB/s. I think you're being shaped. Nothing to do with LW, you're losing all-round internet speed for everything. With a 512 kbps plan you should be getting around 50-55 KB/s (kilobytes) in a speed test. Your upload speed seems consistent though.

Question is where are you losing your speed. Is it equipment or ISP. Since you said the previous week you were getting 50 KB/s, then something has suddenly gone astray. I suspect the problem is your ISP shaping you. Recheck your plan. Are you "sure" you have a 20 GB/mth plan. I suspect you've made a mistake & you're actually only on a much lesser plan like 8 or 10 GB or possibly less.