add performance issues section

Author: espdev

README.md

@@ -72,6 +72,15 @@ fn main() {
 }
  ```
 
+## Performance Issues
+
+Currently, the performance of computation of smoothing splines might be very low for a large data.
+
+The algorithm of sparse matrics mutliplication in sprs crate is not optimized for large diagonal 
+matrices which causes a poor performance of computation of smoothing splines. 
+See [issue](https://github.com/vbarrielle/sprs/issues/184) for details.
+
+
 ## Algorithms and implementations
 
 The crate implementation is based on [ndaray](https://github.com/rust-ndarray/ndarray) and