diff --git a/builtin/log.c b/builtin/log.c
index 099f4f7..e364572 100644
--- a/builtin/log.c
+++ b/builtin/log.c
@@ -1315,7 +1315,7 @@
 		struct object_id *patch_id;
 		if (commit->util)
 			continue;
-		if (commit_patch_id(commit, &diffopt, sha1))
+		if (commit_patch_id(commit, &diffopt, sha1, 0))
 			die(_("cannot get patch id"));
 		ALLOC_GROW(bases->patch_id, bases->nr_patch_id + 1, bases->alloc_patch_id);
 		patch_id = bases->patch_id + bases->nr_patch_id;
diff --git a/patch-ids.c b/patch-ids.c
index 69a14a3..082412a 100644
--- a/patch-ids.c
+++ b/patch-ids.c
@@ -5,7 +5,7 @@
 #include "patch-ids.h"
 
 int commit_patch_id(struct commit *commit, struct diff_options *options,
-		    unsigned char *sha1)
+		    unsigned char *sha1, int diff_header_only)
 {
 	if (commit->parents)
 		diff_tree_sha1(commit->parents->item->object.oid.hash,
@@ -13,13 +13,31 @@
 	else
 		diff_root_tree_sha1(commit->object.oid.hash, "", options);
 	diffcore_std(options);
-	return diff_flush_patch_id(options, sha1, 0);
+	return diff_flush_patch_id(options, sha1, diff_header_only);
 }
 
+/*
+ * When we cannot load the full patch-id for both commits for whatever
+ * reason, the function returns -1 (i.e. return error(...)). Despite
+ * the "cmp" in the name of this function, the caller only cares about
+ * the return value being zero (a and b are equivalent) or non-zero (a
+ * and b are different), and returning non-zero would keep both in the
+ * result, even if they actually were equivalent, in order to err on
+ * the side of safety.  The actual value being negative does not have
+ * any significance; only that it is non-zero matters.
+ */
 static int patch_id_cmp(struct patch_id *a,
 			struct patch_id *b,
-			void *keydata)
+			struct diff_options *opt)
 {
+	if (is_null_sha1(a->patch_id) &&
+	    commit_patch_id(a->commit, opt, a->patch_id, 0))
+		return error("Could not get patch ID for %s",
+			oid_to_hex(&a->commit->object.oid));
+	if (is_null_sha1(b->patch_id) &&
+	    commit_patch_id(b->commit, opt, b->patch_id, 0))
+		return error("Could not get patch ID for %s",
+			oid_to_hex(&b->commit->object.oid));
 	return hashcmp(a->patch_id, b->patch_id);
 }
 
@@ -43,11 +61,13 @@
 			       struct commit *commit,
 			       struct patch_ids *ids)
 {
+	unsigned char header_only_patch_id[GIT_SHA1_RAWSZ];
+
 	patch->commit = commit;
-	if (commit_patch_id(commit, &ids->diffopts, patch->patch_id))
+	if (commit_patch_id(commit, &ids->diffopts, header_only_patch_id, 1))
 		return -1;
 
-	hashmap_entry_init(patch, sha1hash(patch->patch_id));
+	hashmap_entry_init(patch, sha1hash(header_only_patch_id));
 	return 0;
 }
 
@@ -60,7 +80,7 @@
 	if (init_patch_id_entry(&patch, commit, ids))
 		return NULL;
 
-	return hashmap_get(&ids->patches, &patch, NULL);
+	return hashmap_get(&ids->patches, &patch, &ids->diffopts);
 }
 
 struct patch_id *add_commit_patch_id(struct commit *commit,
diff --git a/patch-ids.h b/patch-ids.h
index dea1ecd..0f34ea1 100644
--- a/patch-ids.h
+++ b/patch-ids.h
@@ -13,7 +13,7 @@
 };
 
 int commit_patch_id(struct commit *commit, struct diff_options *options,
-		    unsigned char *sha1);
+		    unsigned char *sha1, int);
 int init_patch_ids(struct patch_ids *);
 int free_patch_ids(struct patch_ids *);
 struct patch_id *add_commit_patch_id(struct commit *, struct patch_ids *);
diff --git a/t/perf/p3400-rebase.sh b/t/perf/p3400-rebase.sh
new file mode 100644
index 0000000..b3e7d52
--- /dev/null
+++ b/t/perf/p3400-rebase.sh
@@ -0,0 +1,36 @@
+#!/bin/sh
+
+test_description='Tests rebase performance'
+. ./perf-lib.sh
+
+test_perf_default_repo
+
+test_expect_success 'setup' '
+	git checkout -f -b base &&
+	git checkout -b to-rebase &&
+	git checkout -b upstream &&
+	for i in $(seq 100)
+	do
+		# simulate huge diffs
+		echo change$i >unrelated-file$i &&
+		seq 1000 >>unrelated-file$i &&
+		git add unrelated-file$i &&
+		test_tick &&
+		git commit -m commit$i unrelated-file$i &&
+		echo change$i >unrelated-file$i &&
+		seq 1000 | tac >>unrelated-file$i &&
+		git add unrelated-file$i &&
+		test_tick &&
+		git commit -m commit$i-reverse unrelated-file$i ||
+		break
+	done &&
+	git checkout to-rebase &&
+	test_commit our-patch interesting-file
+'
+
+test_perf 'rebase on top of a lot of unrelated changes' '
+	git rebase --onto upstream HEAD^ &&
+	git rebase --onto base HEAD^
+'
+
+test_done
diff --git a/t/t6007-rev-list-cherry-pick-file.sh b/t/t6007-rev-list-cherry-pick-file.sh
index 28d4f6b..1408b60 100755
--- a/t/t6007-rev-list-cherry-pick-file.sh
+++ b/t/t6007-rev-list-cherry-pick-file.sh
@@ -207,4 +207,25 @@
 	test_cmp expect actual
 '
 
+# Corrupt the object store deliberately to make sure
+# the object is not even checked for its existence.
+remove_loose_object () {
+	sha1="$(git rev-parse "$1")" &&
+	remainder=${sha1#??} &&
+	firsttwo=${sha1%$remainder} &&
+	rm .git/objects/$firsttwo/$remainder
+}
+
+test_expect_success '--cherry-pick avoids looking at full diffs' '
+	git checkout -b shy-diff &&
+	test_commit dont-look-at-me &&
+	echo Hello >dont-look-at-me.t &&
+	test_tick &&
+	git commit -m tip dont-look-at-me.t &&
+	git checkout -b mainline HEAD^ &&
+	test_commit to-cherry-pick &&
+	remove_loose_object shy-diff^:dont-look-at-me.t &&
+	git rev-list --cherry-pick ...shy-diff
+'
+
 test_done