Extend doc on dense+sparse
diff --git a/doc/TutorialSparse.dox b/doc/TutorialSparse.dox
index fb07ada..1f0be38 100644
--- a/doc/TutorialSparse.dox
+++ b/doc/TutorialSparse.dox
@@ -257,7 +257,14 @@
\code
sm2 = sm1.cwiseProduct(dm1);
dm2 = sm1 + dm1;
+dm2 = dm1 - sm1;
\endcode
+Performance-wise, the adding/subtracting sparse and dense matrices is better performed in two steps. For instance, instead of doing <tt>dm2 = sm1 + dm1</tt>, better write:
+\code
+dm2 = dm1;
+dm2 += sm1;
+\endcode
+This version has the advantage to fully exploit the higher performance of dense storage (no indirection, SIMD, etc.), and to pay the cost of slow sparse evaluation on the few non-zeros of the sparse matrix only.
%Sparse expressions also support transposition: