What is the difference between a 2011 i5, and a more recent i5?
So lets compare the i5 family of CPU on MBP.

The i5 is a brand name for a family of processor, that is why is it still called i5.

Core i5" 2.4 13-Inch (Late 2011) features a 32 nm "Sandy Bridge" 2.4 GHz Intel "Core i5" processor (2435M), with two independent processor "cores" on a single silicon chip with a 3 MB shared level 3 cache.

The 2015 MBP has up to 2.9GHz dual-core Intel Core i5 processor (Turbo Boost up to 3.3GHz) with 3MB shared L3 cache.

Based on this alone the 2015 model is >30% faster.