diff --git a/borg/archiver.py b/borg/archiver.py
index 3ab156951..66ebccc16 100644
--- a/borg/archiver.py
+++ b/borg/archiver.py
@@ -19,7 +19,7 @@ from .helpers import Error, location_validator, format_time, format_file_size, \
     format_file_mode, ExcludePattern, IncludePattern, exclude_path, adjust_patterns, to_localtime, timestamp, \
     get_cache_dir, get_keys_dir, format_timedelta, prune_within, prune_split, \
     Manifest, remove_surrogates, update_excludes, format_archive, check_extension_modules, Statistics, \
-    is_cachedir, bigint_to_int, ChunkerParams, CompressionSpec, have_cython, \
+    is_cachedir, bigint_to_int, ChunkerParams, CompressionSpec, have_cython, is_slow_msgpack, \
     EXIT_SUCCESS, EXIT_WARNING, EXIT_ERROR
 from .logger import create_logger, setup_logging
 logger = create_logger()
@@ -1015,6 +1015,8 @@ Type "Yes I am sure" if you understand this and want to continue.\n""")
         RemoteRepository.remote_path = args.remote_path
         RemoteRepository.umask = args.umask
         update_excludes(args)
+        if is_slow_msgpack():
+            logger.warning("Using a pure-python msgpack! This will result in lower performance.")
         return args.func(args)
 
 
diff --git a/borg/helpers.py b/borg/helpers.py
index c9ffc00b7..63ef4cfae 100644
--- a/borg/helpers.py
+++ b/borg/helpers.py
@@ -43,6 +43,8 @@ if have_cython():
     from . import crypto
     import msgpack
 
+import msgpack.fallback
+
 
 # return codes returned by borg command
 # when borg is killed by signal N, rc = 128 + N
@@ -791,3 +793,7 @@ def int_to_bigint(value):
     if value.bit_length() > 63:
         return value.to_bytes((value.bit_length() + 9) // 8, 'little', signed=True)
     return value
+
+
+def is_slow_msgpack():
+    return msgpack.Packer is msgpack.fallback.Packer
diff --git a/borg/testsuite/helpers.py b/borg/testsuite/helpers.py
index a27b22719..2faa569da 100644
--- a/borg/testsuite/helpers.py
+++ b/borg/testsuite/helpers.py
@@ -7,9 +7,10 @@ import os
 import pytest
 import sys
 import msgpack
+import msgpack.fallback
 
 from ..helpers import adjust_patterns, exclude_path, Location, format_file_size, format_timedelta, IncludePattern, ExcludePattern, make_path_safe, \
-    prune_within, prune_split, get_cache_dir, Statistics, \
+    prune_within, prune_split, get_cache_dir, Statistics, is_slow_msgpack, \
     StableDict, int_to_bigint, bigint_to_int, parse_timestamp, CompressionSpec, ChunkerParams
 from . import BaseTestCase
 
@@ -480,3 +481,14 @@ def test_file_size_precision():
     assert format_file_size(1234, precision=1) == '1.2 kB'  # rounded down
     assert format_file_size(1254, precision=1) == '1.3 kB'  # rounded up
     assert format_file_size(999990000, precision=1) == '1.0 GB'  # and not 999.9 MB or 1000.0 MB
+
+
+def test_is_slow_msgpack():
+    saved_packer = msgpack.Packer
+    try:
+        msgpack.Packer = msgpack.fallback.Packer
+        assert is_slow_msgpack()
+    finally:
+        msgpack.Packer = saved_packer
+    # this assumes that we have fast msgpack on test platform:
+    assert not is_slow_msgpack()