diff --git a/toolz/itertoolz.py b/toolz/itertoolz.py
index c187193f..efbdd57e 100644
--- a/toolz/itertoolz.py
+++ b/toolz/itertoolz.py
@@ -13,9 +13,6 @@
            'sliding_window', 'partition', 'partition_all', 'count', 'pluck')
 
 
-identity = lambda x: x
-
-
 def remove(predicate, seq):
     """ Return those items of sequence for which predicate(item) is False
 
@@ -120,7 +117,9 @@ def _merge_sorted_key(seqs, key):
     heapq.heapify(pq)
 
     # Repeatedly yield and then repopulate from the same iterator
-    while True:
+    heapreplace = heapq.heapreplace
+    heappop = heapq.heappop
+    while len(pq) > 1:
         try:
             while True:
                 # raises IndexError when pq is empty
@@ -129,11 +128,15 @@ def _merge_sorted_key(seqs, key):
                 item = next(it)  # raises StopIteration when exhausted
                 s[0] = key(item)
                 s[2] = item
-                heapq.heapreplace(pq, s)  # restore heap condition
+                heapreplace(pq, s)  # restore heap condition
         except StopIteration:
-            heapq.heappop(pq)  # remove empty iterator
-        except IndexError:
-            return
+            heappop(pq)  # remove empty iterator
+    if pq:
+        # Much faster when only a single iterable remains
+        _, itnum, item, it = pq[0]
+        yield item
+        for item in it:
+            yield item
 
 
 def interleave(seqs, pass_exceptions=()):
@@ -161,7 +164,7 @@ def interleave(seqs, pass_exceptions=()):
         iters = newiters
 
 
-def unique(seq, key=identity):
+def unique(seq, key=None):
     """ Return only unique elements of a sequence
 
     >>> tuple(unique((1, 2, 3)))
@@ -175,11 +178,18 @@ def unique(seq, key=identity):
     ('cat', 'mouse')
     """
     seen = set()
-    for item in seq:
-        tag = key(item)
-        if tag not in seen:
-            seen.add(tag)
-            yield item
+    seen_add = seen.add
+    if key is None:
+        for item in seq:
+            if item not in seen:
+                seen_add(item)
+                yield item
+    else:  # calculate key
+        for item in seq:
+            val = key(item)
+            if val not in seen:
+                seen_add(val)
+                yield item
 
 
 def isiterable(x):
@@ -214,10 +224,11 @@ def isdistinct(seq):
     """
     if iter(seq) is seq:
         seen = set()
+        seen_add = seen.add
         for item in seq:
             if item in seen:
                 return False
-            seen.add(item)
+            seen_add(item)
         return True
     else:
         return len(seq) == len(set(seq))
@@ -528,9 +539,10 @@ def sliding_window(n, seq):
     d = collections.deque(itertools.islice(it, n), n)
     if len(d) != n:
         raise StopIteration()
+    d_append = d.append
     for item in it:
         yield tuple(d)
-        d.append(item)
+        d_append(item)
     yield tuple(d)
 
 
diff --git a/toolz/tests/test_itertoolz.py b/toolz/tests/test_itertoolz.py
index e8cedbc3..d4045e57 100644
--- a/toolz/tests/test_itertoolz.py
+++ b/toolz/tests/test_itertoolz.py
@@ -3,7 +3,7 @@
 from functools import partial
 from toolz.itertoolz import (remove, groupby, merge_sorted,
                              concat, concatv, interleave, unique,
-                             identity, isiterable,
+                             isiterable,
                              mapcat, isdistinct, first, second,
                              nth, take, drop, interpose, get,
                              rest, last, cons, frequencies,
@@ -14,6 +14,10 @@
 from operator import add, mul
 
 
+def identity(x):
+    return x
+
+
 def iseven(x):
     return x % 2 == 0
 
@@ -54,6 +58,7 @@ def test_merge_sorted():
     assert ''.join(merge_sorted('abc', 'abc', 'abc', key=ord)) == 'aaabbbccc'
     assert ''.join(merge_sorted('cba', 'cba', 'cba',
                                 key=lambda x: -ord(x))) == 'cccbbbaaa'
+    assert list(merge_sorted([1], [2, 3, 4], key=identity)) == [1, 2, 3, 4]
 
 
 def test_interleave():