diff --git a/nix/build-all.py b/nix/build-all.py
index ef4b51712a..7b837cb399 100644
--- a/nix/build-all.py
+++ b/nix/build-all.py
@@ -1189,6 +1189,7 @@ if "cgal" in targets:
         pre_compile_subs=(
             [("build/config.h", "HAVE_OBSTACK_VPRINTF 1", "HAVE_OBSTACK_VPRINTF 0")] if "wasm" in flags else []
         ),
+        # Sometimes ftp.gnu.org is very slow, use ftpmirror.gnu.org as a workaround.
         download_url="https://ftp.gnu.org/gnu/gmp/",
         download_name=f"gmp-{GMP_VERSION}.tar.bz2",
     )