In the early days of Youview it was felt that line stability was more important than speed, to help ensure the delivery of "live" TV where buffering was not really an option. Catchup TV like iPlayer wasn't really affected. This drop in speed was really only noticed by those whose line speed was not too great to start with.
Most people who requested through these forums to have the line speed increased did have their speed "returned".
I think now that there is more experience of the TV service this stability problem no longer seems to be such an issue. If your line speed is reduced when you order Youview, just post back on these forums to have your line reset.
You still need a minimum line speed of 5 Mbps to receive any boost channels.