The developers continually work on squeezing a bit more efficiency out of the core math operations, so I'd expect to see an improvement over time. The details of the underlying implementations are proprietary information, but for 8a -> 11b in particular, I'd guess that improvements in multithreading would be a contributing factor.
Take a look at the release notes in the documentation. You'll see performance enhancements in the MATLAB math section. They don't often go into detail (again, that's MathWorks IP), but you'll see certain functions or function groups listed. In particular, I note that 11a says:
Performance Enhancement
- Matrix transpose
- Element-wise single precision functions
- Sparse matrix indexed assignment
- Many linear algebra functions
- Convolution for long vectors and large matrices with conv and conv2
