Loosing 50% of your throughput with wireless is not unusual. The Sky routers are not the best at delivering a good signal so if your adapter is a cheepo one, this will make it even worse.
If you are going to another room on a different floor, it will only get worse. I think you should consider using
Homeplug adapters These will outperform any wireless connection, except possibly Draft N, for both speed and stability. The ones linked are only an example, you should shop around for the best deal. Others on the forum are using these and may make a recommendation for you.
In the mean time, try changing the channel on the router, you may get better reception. Start at 13 and work down.