I have a series of blog posts (that I try to keep updated over time) that list I what I consider to be the more important hotfixes in each cumulative update for the respective branch of code for that version of SQL Server. In order to make it easier for people (and myself) to find these posts, I have put together a list of these posts below.
Performance and Stability Related Fixes in Post-SQL Server 2014 SP1 Builds
Performance and Stability Related Fixes in Post-SQL Server 2014 RTM Builds
Performance and Stability Related Fixes in Post-SQL Server 2012 SP2 Builds
Performance and Stability Related Fixes in Post-SQL Server 2012 SP1 Builds
Performance Related Fixes in Post-SQL Server 2008 R2 Service Pack 2 Builds
Performance Related Fixes in Post-SQL Server 2008 Service Pack 3 Builds
Performance and Stability-Related Fixes in All SQL Server 2008 SP2 Cumulative Updates
The main point of these posts is to try to convince more people to stay up to date with SQL Server cumulative updates, especially with SQL Server 2012 and 2014, where most cumulative updates have had a large number of very significant hotfixes.
For people who don’t believe in installing SQL Server cumulative updates, this Microsoft KB article might help convince you.