pep-550: Add a note why HAMT is better than dict.copy() (#347)

* pep-550: Add a note why HAMT is better than dict.copy()

* pep-550: Drop now unused images

pep-0550.rst

@@ -1041,6 +1041,11 @@ This proposal preserves 100% backwards compatibility.
 Appendix: HAMT Performance
 ==========================
 
+First, while investigating possibilities of how to implement an
+immutable mapping in CPython, we were able to improve the efficiency
+of ``dict.copy()`` up to 5 times: [4]_.  All benchmarks in this
+section were run against the optimized dict.
+
 To assess if HAMT can be used for Execution Context, we implemented
 it in CPython [7]_.
 
@@ -1067,6 +1072,19 @@ considering how well Python dicts are optimized.
 
 Note, that according to [8]_, HAMT design can be further improved.
 
+There is a limitation of Python ``dict`` design which makes HAMT
+the preferred choice for immutable mapping implementation:
+dicts need to be resized periodically, and resize is expensive.
+The ``dict.copy()`` optimization we were  able to do (see [4]_) will
+only work for dicts that had no deleted items.  Dicts that had
+deleted items need to be resized during ``copy()``, which makes it
+much slower.
+
+Because adding and deleting items from LocalContext is a very common
+operation, we would not be able to always use the optimized
+``dict.copy()`` for LocalContext, frequently resorting to use the
+slower version of it.
+
 
 Acknowledgments
 ===============