There will not be much of a performance improvement will Broadwell as the main focus is on lower power consumption for longer battery life. I will assume that the actual average performance increase will not be greater than 6% compared to Haswell CPUs. The last time Intel CPUs had double digit % performance increase was when they released the 2nd generation Sandy Bridge CPU. The next time I expect a double digit % performance increase is when Intel releases Skylake in late 2015.
The biggest performance increase in Broadwell CPUs will be the integrated GPU performance to keep pace with AMD's integrated GPU performance. However, since you are considering purchasing the Lenovo IdeaPad Y50, I am pretty sure you will not be using Intel's...