diff --git a/src/borg/_chunker.c b/src/borg/_chunker.c
index 20941cb07..75599c5b1 100644
--- a/src/borg/_chunker.c
+++ b/src/borg/_chunker.c
@@ -247,7 +247,7 @@ static PyObject *
 chunker_process(Chunker *c)
 {
     uint32_t sum, chunk_mask = c->chunk_mask;
-    size_t n = 0, old_last, min_size = c->min_size, window_size = c->window_size;
+    size_t n, old_last, min_size = c->min_size, window_size = c->window_size;
 
     if(c->done) {
         if(c->bytes_read == c->bytes_yielded)
@@ -282,15 +282,19 @@ chunker_process(Chunker *c)
      */
     c->position += min_size;
     c->remaining -= min_size;
-    n += min_size;
     sum = buzhash(c->data + c->position, window_size, c->table);
     while(c->remaining > c->window_size && (sum & chunk_mask)) {
-        sum = buzhash_update(sum, c->data[c->position],
-                             c->data[c->position + window_size],
-                             window_size, c->table);
-        c->position++;
-        c->remaining--;
-        n++;
+        uint8_t *p = c->data + c->position;
+        uint8_t *stop_at = p + c->remaining - window_size;
+        size_t did_bytes;
+        while (p < stop_at && (sum & chunk_mask)) {
+            sum = buzhash_update(sum, p[0], p[window_size],
+                                 window_size, c->table);
+            p++;
+        }
+        did_bytes = p - (c->data + c->position);
+        c->position += did_bytes;
+        c->remaining -= did_bytes;
         if(c->remaining <= window_size) {
             if(!chunker_fill(c)) {
                 return NULL;
diff --git a/src/borg/testsuite/chunker_slow.py b/src/borg/testsuite/chunker_slow.py
new file mode 100644
index 000000000..2739a735a
--- /dev/null
+++ b/src/borg/testsuite/chunker_slow.py
@@ -0,0 +1,39 @@
+from io import BytesIO
+from binascii import unhexlify
+
+from ..chunker import Chunker
+from ..crypto.low_level import blake2b_256
+from ..constants import *  # NOQA
+from . import BaseTestCase
+
+
+class ChunkerRegressionTestCase(BaseTestCase):
+
+    def test_chunkpoints_unchanged(self):
+        def twist(size):
+            x = 1
+            a = bytearray(size)
+            for i in range(size):
+                x = (x * 1103515245 + 12345) & 0x7FFFFFFF
+                a[i] = x & 0xFF
+            return a
+
+        data = twist(100000)
+
+        runs = []
+        for winsize in (65, 129, HASH_WINDOW_SIZE, 7351):
+            for minexp in (4, 6, 7, 11, 12):
+                for maxexp in (15, 17):
+                    if minexp >= maxexp:
+                        continue
+                    for maskbits in (4, 7, 10, 12):
+                        for seed in (1849058162, 1234567653):
+                            fh = BytesIO(data)
+                            chunker = Chunker(seed, minexp, maxexp, maskbits, winsize)
+                            chunks = [blake2b_256(b'', c) for c in chunker.chunkify(fh, -1)]
+                            runs.append(blake2b_256(b'', b''.join(chunks)))
+
+        # The "correct" hash below matches the existing chunker behavior.
+        # Future chunker optimisations must not change this, or existing repos will bloat.
+        overall_hash = blake2b_256(b'', b''.join(runs))
+        self.assert_equal(overall_hash, unhexlify("b559b0ac8df8daaa221201d018815114241ea5c6609d98913cd2246a702af4e3"))