End-User Experiences of a Residential ISP Bandwidth Bottleneck

General layperson questions: If a 50% bottleneck exists at a certain utilization rate, is it reasonable to expect that speeds would decline 50%, i.e. in proportion to the size of the bottleneck? Do all ISPs employ some form of QoS in the event of congestion?

If I have a 20mbps provisioned line and someone else has one at 10 on my DSLAM, am I twice as likely to draw bottlenecked data as my competitor?

Thanks a lot!