--- a/mercurial/commands.py	Sat Mar 13 08:59:03 2021 +0100
+++ b/mercurial/commands.py	Sun Nov 08 20:17:09 2020 +0100
@@ -1648,6 +1648,14 @@
     if complevel is not None:
         compopts[b'level'] = complevel
 
+    compthreads = ui.configint(
+        b'experimental', b'bundlecompthreads.' + bundlespec.compression
+    )
+    if compthreads is None:
+        compthreads = ui.configint(b'experimental', b'bundlecompthreads')
+    if compthreads is not None:
+        compopts[b'threads'] = compthreads
+
     # Bundling of obsmarker and phases is optional as not all clients
     # support the necessary features.
     cfg = ui.configbool

--- a/mercurial/configitems.py	Sat Mar 13 08:59:03 2021 +0100
+++ b/mercurial/configitems.py	Sun Nov 08 20:17:09 2020 +0100
@@ -866,6 +866,31 @@
 )
 coreconfigitem(
     b'experimental',
+    b'bundlecompthreads',
+    default=None,
+)
+coreconfigitem(
+    b'experimental',
+    b'bundlecompthreads.bzip2',
+    default=None,
+)
+coreconfigitem(
+    b'experimental',
+    b'bundlecompthreads.gzip',
+    default=None,
+)
+coreconfigitem(
+    b'experimental',
+    b'bundlecompthreads.none',
+    default=None,
+)
+coreconfigitem(
+    b'experimental',
+    b'bundlecompthreads.zstd',
+    default=None,
+)
+coreconfigitem(
+    b'experimental',
     b'changegroup3',
     default=False,
 )

--- a/mercurial/utils/compression.py	Sat Mar 13 08:59:03 2021 +0100
+++ b/mercurial/utils/compression.py	Sun Nov 08 20:17:09 2020 +0100
@@ -685,9 +685,11 @@
         # while providing no worse compression. It strikes a good balance
         # between speed and compression.
         level = opts.get(b'level', 3)
+        # default to single-threaded compression
+        threads = opts.get(b'threads', 0)
 
         zstd = self._module
-        z = zstd.ZstdCompressor(level=level).compressobj()
+        z = zstd.ZstdCompressor(level=level, threads=threads).compressobj()
         for chunk in it:
             data = z.compress(chunk)
             if data:

--- a/relnotes/next	Sat Mar 13 08:59:03 2021 +0100
+++ b/relnotes/next	Sun Nov 08 20:17:09 2020 +0100
@@ -5,6 +5,11 @@
  * The `rev-branch-cache` is now updated incrementally whenever changesets
    are added.
 
+ * The new options `experimental.bundlecompthreads` and
+   `experimental.bundlecompthreads.<engine>` can be used to instruct
+   the compression engines for bundle operations to use multiple threads
+   for compression. The default is single threaded operation. Currently
+   only supported for zstd.
 
 == New Experimental Features ==
 

--- a/tests/test-bundle-type.t	Sat Mar 13 08:59:03 2021 +0100
+++ b/tests/test-bundle-type.t	Sun Nov 08 20:17:09 2020 +0100
@@ -201,6 +201,15 @@
   (see 'hg help bundlespec' for supported values for --type)
   [10]
 
+zstd supports threading
+
+  $ hg init test-compthreads
+  $ cd test-compthreads
+  $ hg debugbuilddag +3
+  $ hg --config experimental.bundlecompthreads=1 bundle -a -t zstd-v2 zstd-v2-threaded.hg
+  3 changesets found
+  $ cd ..
+
 #else
 
 zstd is a valid engine but isn't available