I suggest you post your problem on the infinity board and probably get more replies
You've still got the 10 day stabilising period to get through, so the slowdown should be expected. But monitor it and if after 10 you run a speedtest at www.speedtester.bt.com. And it still shows your download speed as slower than your old ADSL2, then get in touch with the help desk in India.
As its a new activation your line speed can vary while is is looking for the best stable speed.
How do you connect, is it wired or wireless.
Also can you do a BT speedtest awired if possible and post the results with the IP profile for your line.
 I will ask the mods to move your post to the Infinity forum,