only sort small dicts and sets in lib.pretty

The sorting caused performance problems when displaying large collections,
even when the result would be truncated.

IPython/lib/pretty.py

@@ -592,10 +592,11 b' def _set_pprinter_factory(start, end, basetype):'
         else:
             step = len(start)
             p.begin_group(step, start)
-            # Like dictionary keys, we will try to sort the items.
-            items = list(obj)
+            # Like dictionary keys, we will try to sort the items if there aren't too many
+            items = obj
+            if not (p.max_seq_length and len(obj) >= p.max_seq_length):
             try:
-                items.sort()
+                    items = sorted(obj)
             except Exception:
                 # Sometimes the items don't sort.
                 pass
@@ -623,9 +624,11 b' def _dict_pprinter_factory(start, end, basetype=None):'
             return p.text('{...}')
         p.begin_group(1, start)
         keys = obj.keys()
+        # if dict isn't large enough to be truncated, sort keys before displaying
+        if not (p.max_seq_length and len(obj) >= p.max_seq_length):
         try:
-            keys.sort()
-        except Exception as e:
+                keys = sorted(keys)
+            except Exception:
             # Sometimes the keys don't sort.
             pass
         for idx, key in p._enumerate(keys):